CBSE Class 9 Physics Force And Laws Of Motion Worksheet

Question. What is Newton's First Law known as? The Law of
A. Friction.
B. Mass.
C. Inertia.
D. Gravity
Answer: Inertia

Question. Motions need an unbalanced net force to maintain
A. True.
B. False
Answer: true

Question. Which has more Inertia? (so need more force to move)
A. Truck.
B. Ant.
C.A cat.
D.A skateboard
Answer: Truck

Question. What is Newton's First Law
A. F=ma
B. Every action has an equal and opposite reaction
C. An object at rest stays at rest, an object in motion stays in motion.
D. Friction
Answer: An object at rest stays at rest, an object in motion stays in motion.

Question. friction is a force that acts
A. in the same direction as the motion
B. opposite to the direction of the motion
C. opposite to the applied forces
D. the same direction as the applied force
Answer: opposite to the direction of the motion

Question. What is the name for a push or pull?
A. A movement.
B. A force.
C. Speed.
D. Density
Answer: B

Question. What can a force do?
A. start a motion
B. stop a motion
C. change the direction of the motion
D. All of the above
Answer: all of the above

Question. Which type of force(s) will cause a change in an object's motion?
A. Gravity.
B. zero net force.
C. balanced forces.
D. unbalanced forces
Answer: Unbalanced force

Question. What kind(s) of objects have inertia?
A. all objects with mass.
B. only objects at rest.
C. only objects in motion.
D. only objects whose motion is being changed
Answer: All objects with mass

Question. Which one needs a greater net force to move, a kilogram of feathers or a kilogram of iron?
A. feathers
B. iron.
C. same.
D. depends on their friction
Answer: Depends on their friction

Question. an object with a zero net force could be moving
A. true.
B. false
Answer: False

Question. an object with a zero net force could be moving in a circle at a constant speed
A. false.
B. true
Answer: True

Question. Which object has more inertia — a Bowling ball or a tennis ball?
Answer: A bowling ball

Question. The passenger in a running bus tends to lean forward , when the bus stops suddenly. This is due to
A. inertia of motion.
B. Inertia in rest
C. Inertia with direction
D. None of the above
Answer: Inertia if motion

Question. The moment the ball was thrown, the ball was also in motion alone with the person who has thrown the ball. This is the example of :-
A. Newton’s second law of motion
B. Newton’s mass law of motion
C. Newton’s first law of motion
D. All of the above
Answer: Newton’s first law of motion.

Question. Why are tyres corrugated and made rough?
Answer: Tyres are corrugated and made rough to increase friction.
(1) This provides better grip on the road.
(2) It also prevents the vehicles from slipping.

Question. Why you get hurt by hitting a stone while when you kick a football it flies away?
Answer: This is because stone is heavier than football and heavier objects offer larger inertia. When we kick a football its mass is less and inertia is also less so force applied by our kick acts on it and hence it shows larger displacement but in case of stone, it has more mass and offers larger inertia. When we kick (action) the stone it exerts an equal and opposite force (reaction) and hence it hurts the foot.

Question. State the law of conservation of momentum.
Answer: (i) Momentum of two bodies before collision is equal to the momentum after collision.
(ii) In an isolated system, the total momentum remains conserved.

Question. Why is it dangerous to jump out from a moving bus?
Answer: While moving in a bus our body is in motion. On jumping out of a moving bus our feet touches the ground and come to rest. While the upper part of our body fall in motion and moves forward due to inertia of motion and hence we can fall in forward direction.
Thus, to avoid this we need to run forward in the direction of bus.

Question. When we hold a bag full of books in our hand steady at some height above the ground, what kind of forces are acting on it?
Answer: The bag full of books is steady. This means its position is fixed. So, it must be under balanced forces. The forces acting on it are :
(i) Weight (or the gravitational pull) downwards,
(ii) Upward force applied by the person holding the bag.

Question. Why all cars are provided with seat belts?
Answer: Sudden movement of the vehicle results in the sudden change in the state of motion of the vehicle when our feet are in contact with it. But the rest of our body opposes this change due to its inertia and tends to remain where it was. Seat belts are provided to protect the passengers from falling backward or forward during such situation.

Question. Why are athletes made to fall either on a cushioned bed or on a sand bed in a high jump athletic event?
Answer: In a high jump athletic event, athletes are made to fall either on a cushioned bed or on a sand bed so as to increase the time of the athlete’s fall to stop after making the jump. This decreases the rate of change of momentum and hence the force.

Question. Which concept is behind the phenomenon- “A boatman pushes the river bank with a bamboo pole to take his boat into the river”.
Answer: When the boatman pushes the river bank with a bamboo pole, the river bank offers an equal and opposite reaction. This reaction helps the boat to move into the river.

Question. Why does a fireman struggle to hold a hose-pipe?
Answer: A fireman has to make a great effort to hold a hosepipe to throw a stream of water on fire to extinguish it. This is because the stream of water rushing through the hose-pipe in the forward direction with a large speed exerts a large force on the hose-pipe in the backward direction.

Question. What happens when a quick jerk is given to a smooth thick cardboard placed on a tumbler with a small coin placed on the cardboard? The coin will fall in the tumbler. Why?
Answer: The coin was initially at rest. When the cardboard moves because of the jerk, the coin tends to remain at rest due to inertia of rest. When the cardboard leaves contact with the coin, the coin falls in the tumbler on account of gravity.

Question. Why is it necessary to bend knees while jumping from greater height?
Answer:
(i) Our feet come to rest at once during the jump.
(ii) Due to the less time a large force acts on the feet.
(iii) If we bend out knees, the time of impact increases. Hence, less force acts on the feet.

Question. Why is it difficult to achieve a zero unbalanced force in practical situations? In practice what happens to a rolling marble? How can we reduce the effect of friction on a marble?
Answer:
(i) It is difficult to achieve a zero unbalanced force because of the presence of the frictional force acting opposite to the direction of motion.
(ii) In practice, the rolling marble stops after travelling some distance.
(iii) The effect of frictional force may be reduced by using a smoother marble and a smoother plane and providing a lubricant.

Question. A batsman has a choice to use heavy or light bat, while facing a fast bowler. He will prefer:
(A) light bat, because handling it is easy
(B) heavy bat, so that he can handle it firmly
(C) heavy bat, because it will recoilless
(D) none of the above.

Answer: C

Question. A man while running at a constant acceleration of 3.5 ms–2, develops a force of 280 N. The mass of man is:
(A) 80 kg
(B) 85 kg
(C) 75 kg
(D) 60 kg

Answer: A

Question. The mass of an aeroplane is 2.5 t. Its engine develops a force of 8750 N before taking off. The acceleration of the aeroplane at the time of take off is :
(A) 3.45 ms^–2
(B) 3.65 ms^–2
(C) 3.50 ms^–2
(D) 3.60 ms^–2

Answer: C

Question. A 8000 kg engine pulls a train of 5 wagons, each of 2000 kg along a horizontal track. If the engine exerts a force of 40,000 N and track offers a friction of 5000 N, then net accelerating force acting on the system is :
(A) 45,000 N
(B) 40,000 N
(C) 35,000 N
(D) none of the above

Answer: C

Question. A body is moving with a constant momentum. The motion of the body is :
(A) uniform velocity
(B) accelerated
(C) de-accelerated
(D) none of the above.

Answer: D

Question. A horse while running at a constant velocity of 15 ms^–1, develops a momentum of 3000 Ns. The mass of horse is:
(A) 180 kg
(B) 300 kg
(C) 200 kg
(D) 250 kg

Answer: C

Question. A boy of mass 30 kg while running at constant velocity has a momentum of 180 Ns. The constant velocity of the boy is :
(A) 3 ms^–1
(B) 6 ms^–1
(C) 18 ms^–1
(D) 12 ms^–1

Answer: B

Question. An electron of mass 9 x 10^–31 kg is moving in a straight line path with a velocity of 6 × 10⁷ ms^–1. The momentum of electron is :
(A) 5.4 × 10^–23 Ns
(B) 5.4 × 10^–24 Ns
(C) 4.5 × 10^–23 Ns
(D) 0.5 × 10^–24 Ns

Answer: A

Question. A vehicle has a mass of 1500 kg. If the vehicle is to be stopped with a negative acceleration of 1.7 ms–2, the force of friction between the vehicle and road is :
(A) –2250 N
(B) –2050 N
(C) –2550 N
(D) none of the above.

Answer: C

Question. A bullet of mass 0.01 kg is fired from a rifle. The bullet takes 0.003 s to move through the barrel and leaves with a velocity of 300 ms^–1. The acceleration acting on the bullet is :
(A) 10,000 ms^–2
(B) 100,000 ms^–2
(C) 1000,000 ms^–2
(D) 1000 ms^–2

Answer: B

Question. By Newton's second law, the physical quantity which can be calculated, if we know the magnitude of force on a given mass is :
(A) velocity
(B) momentum
(C) acceleration
(D) none of the above.

Answer: B

Question. A truck starts from rest and rolls down the hill with a constant acceleration.
It travels 400 m in 20 s. If the mass of truck is 7 metric tonnes, the force acting on it is:
(A) 28,000 N
(B) 14,000 N
(C) 1400 N
(D) 24,000 N

Answer: B

Question. A cracker at rest explodes into two equal parts. These parts will move in :
(A) opposite direction with different velocities
(B) same direction with different velocities
(C) same direction with same velocities
(D) opposite direction with same velocities

Answer: D

More Question..

Multiple Choice Questions

1. Which of the following statement is not correct for an object moving along a straight path in an accelerated motion?
(a) Its speed keeps changing
(b) Its velocity always changes
(c) It always goes away from the earth
(d) A force is always acting on it

2. The forces of action and reaction are
(a) always equal only
(b) always equal and opposite
(c) always equal but in same direction
(d) always unequal and opposite.

3. According to the third law of motion, action and reaction
(a) always act on the same body
(b) always act on different bodies in opposite directions
(c) have same magnitude and directions
(d) act on either body at normal to each other

4. The action and reaction forces at
(a) on different bodies always
(b) on some body always
(c) on same body, sometimes
(d) on different bodies, sometimes

5. A goalkeeper in a game of football pulls his hands backwards after holding the ball shot at the goal. This enables the goal keeper to
(a) exert larger force on the ball
(b) reduce the force exerted by the ball on hands
(c) increase the rate of change of momentum
(d) decrease the rate of change of momentum

6. The inertia of an object tends to cause the object
(a) to increase its speed
(b) to decrease its speed
(c) to resist any change in its state of motion
(d) to decelerate due to friction

7. Principle of conservation of linear momentum is deduced from
(a) Newton’s first law
(b) Newton’s second law
(c) Newton’s third law
(d) none of the above

8. The function of mud guards is based on
(a) inertia of rest
(b) inertia of direction
(c) inertia of motion
(d) none of the above

9. The force of action and reaction
(a) always cancel each other
(b) never cancel
(c) cancel sometimes
(d) cannot say

10. A passenger in a moving train tosses a coin which falls behind him. It means that motion of the train is
(a) accelerated
(b) uniform
(c) retarded
(d) along circular tracks

11. An object of mass 2 kg is sliding with a constant velocity of 4 m s–1 on a frictionless horizontal table. The force required to keep the object moving with the same velocity is
(a) 32N
(b) 0 N
(c) 2 N
(d) 8 N

12. Rocket works on the principle of conservation of
(a) mass
(b) energy
(c) momentum
(d) velocity

13. A water tanker filled up to 2/3 of its height is moving with a uniform speed. On sudden application of the brake, the water in the tank would
(a) move backward
(b) move forward
(c) be unaffected
(d) rise upwards

14. Inertia of a body in linear motion is measured by its
(a) mass
(b) momentum
(c) velocity
(d) none of the above

15. What mass of a body can attain an acceleration of 5m/s2 under a force of 250 N?
(a) 5 kg
(b) 250 kg
(c) 50 kg
(d) 10 kg

Question. What is the direction of momentum?
Answer. The direction of momentum is the same as that of velocity.

Question. Which type of force brings an object in motion?
Answer. Unbalanced force.

Question. Define force.
Answer. It is a push or pull on an object that produces acceleration in the body on which it acts.

Question. What is balanced force?
Answer. When forces acting on a body from the opposite direction do not change the state of rest or of motion of an object, such forces are called balanced forces.

Question. No force is required to move an object with a constant velocity. Why?
Answer. For an object moving with a constant velocity, a = 0,
so F = ma = 0

Question. Why do the passengers fall backward when a bus accelerates from rest?
Answer. The passengers fall backward when a bus accelerates from rest due to their inertia. The passengers tend to remain in their state of rest even when the bus starts moving.

Question. What are the changes possible on an object at rest if we apply on it?
(a) A balanced force?
(b) An unbalanced force?
Answer.
(a) Object may changes its shape or size.
(b) Objects changes its speed, velocity or accelerates.

Question. What happens when you shake a wet piece of cloth? Explain your observation.
Answer. When a wet piece of cloth is shaken, small droplets of water fall down. This is because in the beginning both water and the piece of cloth were at rest. When he cloth is moved, the water in it tends to remain in the state of rest due to inertia of rest. As a result, the droplets of water fall down and the clothes dry quickly.

Question. State all 3 Newton’s law of motion.
Answer. Newton’s I law of motion : An object remains in a state of rest or of uniform motion in a straight line unless acted upon by an external unbalanced force. Newton’s II law of motion : The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force. Newton’s III law of
motion : To every action, there is
an equal and opposite reaction and they act on two different bodies.

Question. Write the effect of force.
or
What change will be in a body by force?
Answer. Force can produce three effects :
(a) It can change the magnitude of velocity of an object (i.e. to make the object move faster or slower).
(b) It can change the direction of motion of an object.
(c) It can change the shape and size of an object.

Question. What do you understand by the gravitational unit of force?
Answer.A gravitational unit of force is defined as that force which produces an acceleration equal to the acceleration due to gravity in a body of one unit mass. Gram-weight and kilogram-weight are the gravitational unit of force.

Question.Which of the following has more inertia: (a) a rubber ball and a stone of the same size? (b) a bicycle and a train? (c) a five-rupee coin and a one-rupee coin?
Answer.
Since inertia is dependent on the mass of the object, the object with the greater mass will hold greater inertia. The following objects hold greater inertia because of their mass.
Stone
Train
Five-Rupee coin

Question. In the following example, try to identify the number of times the velocity of the ball changes: “A football player kicks a football to another player of his team who kicks the football towards the goal. The goalkeeper of the opposite team collects the football and kicks it towards a player of his own team”. Also identify the agent supplying the force in each case.
Answer.
Let the two football teams be team A and team B respectively. Initially, the football is at rest. Now, the football is kicked by a player on team A to another player of team A. (The velocity of the ball has changed 1 time so far). This change is brought on by the force applied by the player who kicked the ball.
The football is now kicked by the other team A player towards the goal. (The velocity of the ball has changed 2 times so far). This change is also brought on due to the force applied by the team A player who kicked the ball.
The goalkeeper of team B stops the ball, bringing it to rest. (the velocity of the ball has changed 3 times so far). This change is brought on by the force applied by the goalkeeper of team B to stop the ball.
Finally, the goalkeeper of team B kicks the ball towards another player of team B. the velocity of the ball changes for a final time, bringing the total count to 4. This change is also brought on by the goalkeeper of team B, who applies the force to kick the ball forward.
Therefore, the velocity of the ball changes 4 times in this example.

Question. Explain why some of the leaves may get detached from a tree if we vigorously shake its branch.
Answer.
When the branch of the tree is shaken, the branch moves in a to-and-fro motion. However, the inertia of the leaves in attached to the branch resists the motion of the branch. Therefore, the leaves that are weakly attached to the branch fall off due to inertia whereas the leaves that are firmly attached to the branch remain attached.

Question. Why do you fall in the forward direction when a moving bus brakes to a stop and fall backwards when it accelerates from rest?
Answer.
Initially, when the bus accelerates in a forward direction from a state of rest, the passengers experience a force exerted on them in the backward direction due to their inertia opposing the forward motion.
Once the bus starts moving, the passengers are in a state of motion in the forward direction. When the brakes are applied, the bus moves towards a position of rest. Now, a force in the forward direction is applied on the passengers because their inertia resists the change in the motion of the bus. This causes the passengers to fall forwards when the brakes are applied.

Question. If action is always equal to the reaction, explain how a horse can pull a cart.
Answer.
When the horse walks in the forward direction (with the cart attached to it), it exerts a force in the backward direction on the Earth. An equal force in the opposite direction (forward direction) is applied on the horse by the Earth. This force moves the horse and the cart forward.
The velocity at which the horse can move by applying a force on the earth depends on the mass of the horse (and the cart attached to it). The heavier the cart, the slower the motion of the horse (for a given amount of force applied by the horse on the Earth). If the cart is too heavy, the force exerted by the horse on the Earth will be insufficient to even overcome the force of inertia. In this case, the horse will not be able to pull the cart.

Question. Explain, why is it difficult for a fireman to hold a hose, which ejects large amounts of water at a high velocity.
Answer.
For the hose to eject water at high velocities, a force must be applied on the water (which is usually done with the help of a pump or a motor). Now, the water applies an equal and opposite force on the hose. For the fireman to hold this hose, he must apply a force on it to overcome the force applied on the hose by the water. The higher the quantity and velocity of the water coming out of the hose, the greater the force that must be applied by the fireman to hold it steady.

Question. From a rifle of mass 4 kg, a bullet of mass 50 g is fired with an initial velocity of 35 m s^–1. Calculate the initial recoil velocity of the rifle.
Answer.
Given, the Bullet’s mass (m₁) = 50 g
The rifle’s mass (m₂) = 4kg = 4000g
Initial velocity of the fired bullet (v₁) = 35 m/s
Let the recoil velocity be v₂.
Since the rifle was initially at rest, the initial momentum of the rifle = 0
Total momentum of the rifle and bullet after firing = m₁v₁ + m₂v₂
As per the law of conservation of momentum, the total momentum of the rifle and the bullet after firing = 0 (same as initial momentum)
Therefore, m₁v₁ + m₂v₂ = 0

Therefore, the recoil velocity of the rifle is 0.4375 meters per second in the direction opposite to the trajectory of the bullet (backward direction).

Question. Two objects of masses 100 g and 200 g are moving along the same line and direction with velocities of 2 ms^–1 and 1 ms^–1, respectively. They collide and after the collision, the first object moves at a velocity of 1.67 ms^–1. Determine the velocity of the second object.
Answer.
Assuming that the first object is object A and the second one is object B, it is given that:
Mass of A (m₁) = 100g
Mass of B (m₂) = 200g
Initial velocity of A (u₁) = 2 m/s
Initial velocity of B (u₂) = 1 m/s
Final velocity of A (v₁) = 1.67 m/s
Final velocity of B (v₂) =?
Total initial momentum = Initial momentum of A + initial momentum of B
= m₁u₁ + m₂u₂
= (100g) × (2m/s) + (200g) × (1m/s) = 400 g.m.sec^-1
As per the law of conservation of momentum, the total momentum before collision must be equal to the total momentum post collision.

v₂ = 1.165 m/s
Therefore, the velocity of object B after the collision is 1.165 meters per second.

Question. A bullet of mass 100 g is fired from a gun of mass 20 kg with a velocity of 100 msl. Calculate the velocity of recoil of the gun.
Answer.
Mass of bullet, m = 100 g – 1000 kg
Velocity of bullet, u = 100 ms^–1
Mass of gun, M = 20 kg
Let recoil velocity of gun = V
Step 1. Before firing, the system (gun + bullet) is at rest, therefore, initial momentum of the system = 0
Final momentum of the system
= momentum of bullet + momentum of gun
= mu + MV = 10
1 × 100 + 20 V
V = 10 + 20 V
Step 2. Apply law of conservation of momentum Final momentum = Initial momentum
i.e. 10 + 20 V = 0
20 V = 10
or V = – 0.5 ms^–1
Negative sign shows that the direction of recoil velocity of the gun is opposite to the direction of the velocity of the bullet.

Question. Define force. What are different types forces?
Answer. Force : It is a push or pull on an object that produces acceleration in the body on which it acts. The S.I. unit of force is Newton.
Types of forces :
Balanced force : When the forces acting on a body from the opposite direction do not change the state of rest or of motion of an object, such forces are called balanced forces.
Unbalanced force : When two opposite forces acting on a body move a body in the direction of the greater force or change the state of rest, such forces are called as unbalanced forces.
Frictional force : Force of friction is the force that always opposes the motion of object.

Question. Why are road accidents at high speeds very much worse than accidents at low speeds?
Answer. The time of impact of vehicles is very small at high speed. So, they exert very large forces on each other.
Hence, road accidents at high speeds are highly fatal.

Question. Name two factors which determine the momentum of a body.
Answer. Two factors on which momentum of a body depend is mass and velocity. Momentum is directly proportional to the mass and velocity of the body.

Question. What decides the rate of change of momentum of an object?
Answer. The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of force.

Question. Why do athletes run some distance before jumping?
Answer. Athlete has the inertia of motion and thus continues to move past the line.

Question. Is force a scalar quantity or a vector quantity?
Answer. Force is a vector quantity. It has both magnitude and direction.

Question. Why action and reaction do not cancel each other?
Answer. Action and reaction act simultaneous but on different objects. Hence, they do not cancel each other.

Question. What is inertia? Explain different types of inertia.
Answer. Inertia : The natural tendency of an object to resist change in their state of rest or of motion is called inertia. The mass of an object is a measure of itsinertia. Its S.I. unit is kg.
Types of inertia :
Inertia of rest : The object remain in rest unless acted upon by an external unbalanced force.
Inertia of motion : The object in the state of uniform motion will continue to remain in motion with same speed and direction unless external force is not appliedon it.

Question. What is the acceleration produced by a force of 5 N exerted on an object of mass 10 kg?
Answer.
Here F = 5 N; m = 10 kg; a = ?
Now F = ma or a = F/m
a = 0.5 ms^–2

Question. Which would require greater force : accelerating a 10 g mass at 5 ms^–2 or 20 g mass at 2 ms^–2?
Answer.
In first case m₁ = 10 g = kg = 0.010 kg;
Now al = 5 ms^–2 ; F₁ = ?
F₁ = m₁a₁ = 0.010 × 5
F₁ = 0.050 Newton
In second case, m₂ = 20 g = 0.020 kg
a₂ = 2 ms^–2; F₂ = ?
Now F₂ = m₂a₂ = 0.020 × 2
or F₂ = 0.04 Newton
We find that F₁ > F₂, hence more force is required to accelerate 10 g at 5 ms^–2 than accelerating 20 g at 2 ms^–2.

Question. State Newton’s second law of motion.
Answer. The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force.

Question. An object experiences a net zero external unbalanced force. Is it possible for the object to be travelling with a non-zero velocity? If yes, state the conditions that must be placed on the magnitude and direction of the velocity. If no, provide a reason.
Answer. Yes, it is possible. An object moving in some direction with constant velocity will continue in its state of motion as long as there are no external unbalanced forces acting on it. In order to change the motion of the object, some external unbalanced force must act upon it.

Question. When a carpet is beaten with a stick, dust comes out of it. Explain.
Answer. When the carpet is beaten with a stick, the stick exerts a force on the carpet which sets it in motion. The inertia of the dust particles residing on the carpet resists the change in the motion of the carpet. Therefore, the forward motion of the carpet exerts a backward force on the dust particles, setting them in motion in the opposite direction. This is why the dust comes out of the carpet when beaten.

Question. Why is it advised to tie any luggage kept on the roof of a bus with a rope?
Answer. When some luggage is placed on the roof of a bus which is initially at rest, the acceleration of the bus in the forward direction will exert a force (in the backward direction) on the luggage. In a similar manner, when a bus which is initially in a state of motion suddenly comes to rest due to the application of brakes, a force (in the forward direction) is exerted on the luggage.
Depending on the mass of the luggage and the magnitude of the force, the luggage may fall off the bus due to inertia. Tying up the luggage will secure its position and prevent it from falling off the bus.

Question. A batsman hits a cricket ball which then rolls on a level ground. After covering a short distance, the ball comes to rest. The ball slows to a stop because (a) the batsman did not hit the ball hard enough. (b) velocity is proportional to the force exerted on the ball. (c) there is a force on the ball opposing the motion. (d) there is no unbalanced force on the ball, so the ball would want to come to rest.
Answer. When the ball rolls on the flat surface of the ground, its motion is opposed by the force of friction (the friction arises between the ground and the ball). This frictional force eventually stops the ball. Therefore, the correct answer is (c).
If the surface of the level ground is lubricated (with oil or some other lubricant), the friction that arises between the ball and the ground will reduce, which will enable the ball to roll for a longer distance.

Question. A truck starts from rest and rolls down a hill with a constant acceleration. It travels a distance of 400 m in 20 s. Find its acceleration. Find the force acting on it if it’s mass is 7 tonnes (Hint: 1 tonne = 1000 kg.)
Answer.
Given, distance covered by the truck (s) = 400 meters
Time taken to cover the distance (t) = 20 seconds
Initial velocity of the truck (u) = 0 (since it starts from a state of rest)

Question. A stone of 1 kg is thrown with a velocity of 20 ms-1 across the frozen surface of a lake and comes to rest after travelling a distance of 50 m. What is the force of friction between the stone and the ice?
Answer.
Given, Mass of the stone (m) = 1kg
Initial velocity (u) = 20m/s
Terminal velocity (v) = 0 m/s (the stone reaches a position of rest)
Distance travelled by the stone (s) = 50 m

Question. An 8000 kg engine pulls a train of 5 wagons, each of 2000 kg, along a horizontal track. If the engine exerts a force of 40000 N and the track offers a friction force of 5000 N, then calculate: (a) the net accelerating force and (b) the acceleration of the train
Answer.
(a) Given, force exerted by the train (F) = 40,000 N
Force of friction = -5000 N (the negative sign indicates that the force is applied in the opposite direction)
Therefore, the net accelerating force = sum of all forces = 40,000 N + (-5000 N) = 35,000 N

(b) Total mass of the train = mass of engine + mass of each wagon = 8000kg + 5 × 2000kg
The total mass of the train is 18000 kg.
As per the second law of motion, F = ma (or: a = F/m)
Therefore, acceleration of the train = (net accelerating force) / (total mass of the train)
= 35,000/18,000 = 1.94 ms^-2
The acceleration of the train is 1.94 m.s^-2.

Question. An automobile vehicle has a mass of 1500 kg. What must be the force between the vehicle and road if the vehicle is to be stopped with a negative acceleration of 1.7 ms^-2?
Answer.
Given, mass of the vehicle (m) = 1500 kg
Acceleration (a) = -1.7 ms^-2
As per the second law of motion, F = ma
F = 1500kg × (-1.7 ms^-2) = -2550 N
Therefore, a force of 2550 N must act on the vehicle in a direction opposite to that of its motion.

Question. What is the momentum of an object of mass m, moving with a velocity v?
(a) (mv)² (b) mv² (c) ½ mv² (d) mv
Answer.
Since momentum is defined as the product of mass and velocity, the correct answer is (d), mv.

Question. Using a horizontal force of 200 N, we intend to move a wooden cabinet across a floor at a constant velocity. What is the friction force that will be exerted on the cabinet?
Answer.
Since the velocity of the cabinet is constant, its acceleration must be zero. Therefore, the effective force acting on it is also zero. This implies that the magnitude of opposing frictional force is equal to the force exerted on the cabinet, which is 200 N. Therefore, the total friction force is -200 N.

Question. Two objects, each of mass 1.5 kg, are moving in the same straight line but in opposite directions. The velocity of each object is 2.5 ms^-1 before the collision during which they stick together. What will be the velocity of the combined object after collision?
Answer.
Given, mass of the objects (m₁ and m₂) = 1.5kg
Initial velocity of the first object (u₁) = 2.5 m/s
Initial velocity of the second object which is moving in the opposite direction (u₂) = -2.5 m/s
When the two masses stick together, the resulting object has a mass of 3 kg (m₁ + m₂)
Velocity of the resulting object (v) =?
As per the law of conservation of momentum, the total momentum before the collision is equal to the total momentum after the collision.
Total momentum before the collision = m₁u₁ + m₂u₂
= (1.5kg) (2.5 m/s) + (1.5 kg) (-2.5 m/s) = 0
Therefore, total momentum after collision = (m₁+m₂) v = (3kg) v = 0
Therefore v = 0
This implies that the object formed after the collision has a velocity of 0 meters per second.

Question. According to the third law of motion when we push on an object, the object pushes back on us with an equal and opposite force. If the object is a massive truck parked along the roadside, it will probably not move. A student justifies this by answering that the two opposite and equal forces cancel each other. Comment on this logic and explain why the truck does not move.
Answer.
Since the truck has a very high mass, the static friction between the road and the truck is high. When pushing the truck with a small force, the frictional force cancels out the applied force and the truck does not move. This implies that the two forces are equal in magnitude but opposite in direction (since the person pushing the truck is not displaced when the truck doesn’t move). Therefore, the student’s logic is correct.

Question. A hockey ball of mass 200 g travelling at 10 ms^–1 is struck by a hockey stick so as to return it along its original path with a velocity at 5 ms^–1. Calculate the magnitude of change of momentum occurred in the motion of the hockey ball by the force applied by the hockey stick.
Answer.
Given, mass of the ball (m) = 200g
Initial velocity of the ball (u) = 10 m/s
Final velocity of the ball (v) = 5m/s
Initial momentum of the ball = mu = 200g × 10 ms^-1 = 2000 g.m.s^-1
Final momentum of the ball = mv = 200g × 5 ms^-1 = 1000 g.m.s^-1
Therefore, the change in momentum (mv – mu) = 1000 g.m.s^-1 – 2000 g.m.s^-1 = -1000 g.m.s^-1
This implies that the momentum of the ball reduces by 1000 g.m.s^-1 after being struck by the hockey stick.

Question. A bullet of mass 10 g travelling horizontally with a velocity of 150 m s^–1 strikes a stationary wooden block and comes to rest in 0.03 s. Calculate the distance of penetration of the bullet into the block. Also calculate the magnitude of the force exerted by the wooden block on the bullet.
Answer.
Given, mass of the bullet (m) = 10g (or 0.01 kg)
Initial velocity of the bullet (u) = 150 m/s
Terminal velocity of the bullet (v) = 0 m/s
Time period (t) = 0.03 s
To find the distance of penetration, the acceleration of the bullet must be calculated.

Therefore, force exerted by the wooden block on the bullet (F) = 0.01kg × (-5000 ms^-2)
= -50 N

This implies that the wooden block exerts a force of magnitude 50 N on the bullet in the direction that is opposite to the trajectory of the bullet.

Question. An object of mass 1 kg travelling in a straight line with a velocity of 10 ms^–1collides with, and sticks to, a stationary wooden block of mass 5 kg. Then they both move off together in the same straight line. Calculate the total momentum just before the impact and just after the impact. Also, calculate the velocity of the combined object.
Answer.
Given, mass of the object (m1) = 1kg
Mass of the block (m₂) = 5kg
Initial velocity of the object (u₁) = 10 m/s
Initial velocity of the block (u₂) = 0
Mass of the resulting object = m₁ + m₂ = 6kg
Velocity of the resulting object (v) =?
Total momentum before the collision = m₁u₁ + m₂u₂ = (1kg) × (10m/s) + 0 = 10 kg.m.s^-1
As per the law of conservation of momentum, the total momentum before the collision is equal to the total momentum post the collision. Therefore, the total momentum post the collision is also 10 kg.m.s^-1
Now, (m₁ + m₂) × v = 10kg.m.s^-1

The resulting object moves with a velocity of 1.66 meters per second.

Question. Explain why- An inflated balloon lying on the surface of a floor moves forward when pierced with a pin.
Answer. The momentum of the inflated balloon is zero before it is pierced with a pin. Air comes out with a speed in the backward direction from balloon after it is pierced with a pin. The balloon moves in the forward direction to conserve the momentum.

Question. Explain our walking in terms of Newton’s third law of motion.
Answer. When we walk on ground, our foot pushes the ground backward and in return the ground pushes our foot forward. The forward reaction exerted by the ground on our foot makes us walk forward.

Question. There are three solid balls, made up of aluminium,steel and wood of the same shape and volume. Which of them would have highest inertia? Why?
Answer. Steel ball because the density of steel is maximum. For the same shape and volume, steel ball will have maximum mass.

Question. State Newton’s three laws of motion.
Answer. Sir Isssac Newton further studied the idea of Galileo’s on force and motion and presented three laws of motion. These laws are as follows :
(i) First Law : A body remains in resting position unless it is not introduced with an unbalanced external force.
(ii) Second Law : The rate of change of momentum of a body is directly proportional to the applied unbalanced force and change takes place in the direction of the force.
(iii) Third law : Action and reaction are equal and opposite and they act on different bodies.

Question. What is frictional force?
Answer. The force that always opposes the motion of object is called force of friction.

Question. Give example to show the effects of force.
Answer. (i) Place a ball in the ground. Kick it with your foot. The ball starts moving. The ball moves because of the force applied to it.
(ii) If ball is coming towards you, you can kick it in any direction. The direction of motion of the ball changes because of the force applied to it.
(iii) Place a rubber on the ground. Press it with your foot. It is found that the ball is no longer round but takes the shape of an egg, i.e. it is oblong. The shape of the ball has changed because of the force applied on the ball.

Question. Why we tend to get thrown to one side when a motorcar makes a sharp turn at a high speed?
Answer. We tend to get thrown to one side when a motorcar makes a sharp turn at a high speed due to law of inertia. When we are sitting in moving car on a straight road, we tend to continue in our straight-line motion. But when an unbalanced force is applied on car to change the direction of motion, we slip to one side of the seat due to the inertia of our body.

Question. Why do fielders pull their hand gradually with the moving ball while holding a catch?
Answer. While catching a fast moving cricket ball, a fielder on the ground pulls his hands backwards with the moving ball. This is done so that the fielder increases the time during which the high velocity of the moving ball decreases to zero. Thus, the acceleration of the ball is decreased and therefore, the impact of catching the fast moving ball is reduced.

Question. What is inertia?
Answer.The natural tendency of an object to resist a change in their state of rest or of uniform motion is called inertia.

Question. It is required to increase the velocity of a scooter of mass 80 kg from 5 to 25 ms–2 in 2 seconds. Calculate the force required.
Answer.
Given : m = 80 kg,
u = 5 ms^–2
v = 25 ms^–2
and t = 2 s
Now acceleration a = change in velocity time
= v – u/t
= 25 – 5/2 = 10 ms^–2
Force = mass × acceleration of F
= ma
Therefore, F = 80 × 10 = 800 N

Question. State Newton’s first law of motion.
Answer. An object remains in a state of rest or of uniform motion in a straight line unless acted upon by an external unbalanced force.

Question. How is force represented graphically?
Answer. Force is a vector quantity. So, force is represented by a line with an arrow head.
(i) The head of arrow represents the direction of the force.
(ii) The length of the line is proportional to the magnitude of the force.
(iii) This is done by choosing a convenient scale.
(iv) For example : A force of 10 N can be represented by a line with an arrow head.
(v) The length of the line depends upon the scale chosen. So, if a scale of 1 cm = 2 N is chosen, the force of 10 N is represented by a line of 5 cm in length.

Question. What are the disadvantages of friction?
or
Why friction is considered wasteful?
Answer. Friction is considered wasteful because :
(1) Friction leads to a loss of energy. Therefore, it reduces the efficiency of machines.
(2) Friction cause wear and tear of machine’s parts.

Question. The following is the distance-time table of an object in motion:

(a) What conclusion can you draw about the acceleration? Is it constant, increasing, decreasing, or zero?
(b) What do you infer about the forces acting on the object.
Answer. (a) The distance covered by the object at any time interval is greater than any of the distances covered in previous time intervals. Therefore, the acceleration of the object is increasing.
(b) As per the second law of motion, force = mass × acceleration. Since the mass of the object remains constant, the increasing acceleration implies that the force acting on the object is increasing as well

Question. Two persons manage to push a motorcar of mass 1200 kg at a uniform velocity along a level road. The same motorcar can be pushed by three persons to produce an acceleration of 0.2 m s^-2. With what force does each person push the motorcar? (Assume that all persons push the motorcar with the same muscular effort)
Answer. Given, mass of the car (m) = 1200kg
When the third person starts pushing the car, the acceleration (a) is 0.2ms^-2. Therefore, the force applied by the third person (F = ma) is given by:
F = 1200kg × 0.2 ms^-2 = 240N

The force applied by the third person on the car is 240 N. Since all 3 people push with the same muscular effort, the force applied by each person on the car is 240 N.

Question. A hammer of mass 500 g, moving at 50 m s^-1, strikes a nail. The nail stops the hammer in a very short time of 0.01 s. What is the force of the nail on the hammer?
Answer. Given, mass of the hammer (m) = 500g = 0.5kg
Initial velocity of the hammer (u) = 50 m/s
Terminal velocity of the hammer (v) = 0 (the hammer is stopped and reaches a position of rest).
Time period (t) = 0.01s

a = -5000ms^-2
Therefore, the force exerted by the hammer on the nail (F = ma) can be calculated as:
F = (0.5kg) * (-5000 ms^-2) = -2500 N
As per the third law of motion, the nail exerts an equal and opposite force on the hammer. Since the force exerted on the nail by the hammer is -2500 N, the force exerted on the hammer by the nail will be +2500 N.

Question. A motorcar of mass 1200 kg is moving along a straight line with a uniform velocity of 90 km/h. Its velocity is slowed down to 18 km/h in 4 s by an unbalanced external force. Calculate the acceleration and change in momentum. Also calculate the magnitude of the force required.
Answer. Given, mass of the car (m) = 1200kg
Initial velocity (u) = 90 km/hour = 25 meters/sec
Terminal velocity (v) = 18 km/hour = 5 meters/sec
Time period (t) = 4 seconds

Therefore, the acceleration of the car is -5 ms^-2.
Initial momentum of the car = m × u = (1200kg) × (25m/s) = 30,000 kg.m.s^-1
Final momentum of the car = m × v = (1200kg) × (5m/s) = 6,000 kg.m.s^-1
Therefore, change in momentum (final momentum – initial momentum) = (6,000 – 30,000) kg.m.s^-1
= -24,000 kg.m.s^-1
External force applied = mass of car × acceleration = (1200kg) × (-5 ms^-2) = -6000N
Therefore, the magnitude of force required to slow down the vehicle to 18 km/hour is 6000 N

Question. An object of mass 100 kg is accelerated uniformly from a velocity of 5 ms–1 to 8 ms^–1 in 6 s. Calculate the initial and final momentum of the object. Also, find the magnitude of the force exerted on the object.
Answer. Given, mass of the object (m) = 100kg
Initial velocity (u) = 5 m/s
Terminal velocity (v) = 8 m/s

Time period (t) = 6s
Now, initial momentum (m × u) = 100kg × 5m/s = 500 kg.m.s^-1
Final momentum (m × v) = 100kg × 8m/s = 800 kg.m.s^-1

Therefore, the object accelerates at 0.5 ms^-2. This implies that the force acting on the object (F = ma) is equal to:
F = (100kg) × (0.5 ms^-2) = 50 N
Therefore, a force of 50 N is applied on the 100kg object, which accelerates it by 0.5 ms^-2.

Question. Akhtar, Kiran, and Rahul were riding in a motorcar that was moving with a high velocity on an expressway when an insect hit the windshield and got stuck on the windscreen. Akhtar and Kiran started pondering over the situation. Kiran suggested that the insect suffered a greater change in momentum as compared to the change in momentum of the motorcar (because the change in the velocity of the insect was much more than that of the motorcar). Akhtar said that since the motorcar was moving with a larger velocity, it exerted a larger force on the insect. And as a result the insect died. Rahul while putting an entirely new explanation said that both the motorcar and the insect experienced the same force and a change in their momentum. Comment on these suggestions.
Answer. Kiran’s suggestion is correct. The mass of the insect is very small when compared to the mass of the car. As per the law of conservation of momentum, the total momentum before the collision between the insect and the car is equal to the total momentum after the collision. Therefore, the change in the momentum of the insect is much greater than the change in momentum of the car (since force is proportional to mass).
Akhtar’s suggestion is also correct. Since the mass of the car is very high, the force exerted on the insect during the collision is also very high.
Rahul’s suggestion is partially correct. As per the third law of motion, the force exerted by the insect on the car is equal and opposite to the force exerted by the car on the insect. However, Rahul’s suggestion that the change in the momentum is the same contradicts the law of conservation of momentum.

Question. How much momentum will a dumb-bell of mass 10 kg transfer to the floor if it falls from a height of 80 cm? Take its downward acceleration to be 10 ms^–2.
Answer. Given, mass of the dumb-bell (m) = 10kg
Distance covered (s) = 80cm = 0.8m
Initial velocity (u) = 0 (it is dropped from a position of rest)
Acceleration (a) = 10ms^-2
Terminal velocity (v) =?
Momentum of the dumb-bell when it hits the ground = mv

v = 4 m/s
The momentum transferred by the dumb-bell to the floor = (10kg) × (4 m/s) = 40 kg.m.s^-1

First Law of Motion

Frictional force
The force that opposes relative motion is called friction. It arises between the surfaces in contact.
Example: When we try to push a table and it does not move is because it is balanced by the frictional force.

Galileo’s Observation
• He observed the motion of objects on an inclined plane.
• When a marble is rolled down an inclined plane its velocity increases.

Galileo’s Arguments
• When a marble is rolled down from the left – It will go up on the opposite side up to the same height at which it is dropped down.

• If the inclination of planes is equal – The marble would travel equal distances while climbing up as travelled while rolling down.

• If we decrease the angle of inclination of the right plane – The marble would travel further until it reaches its original height.

• If the right side plane is made flat – Marble would travel forever to achieve the same height.

Galileo's Inference
• We need an unbalanced force to change the motion of the marble but no force is required when the marble is moving uniformly. In other words, objects move at a constant speed if no force acts upon them

First Law of Motion
A body continues to be in the state of rest or uniform motion in a straight line unless acted upon by an external unbalanced force. First Law is also called the Law of Inertia.

Inertia
Basically, all objects have a tendency to resist the change in the state of motion or rest. This tendency is called inertia. All bodies do not have the same inertia. Inertia depends on the mass of a body. Mass of an object is the measure of its inertia. More the mass → more inertia and vice versa.

Types of Inertia
It is of three types:
▪ Inertia of Rest
▪ Inertia of Motion
▪ Inertia of direction
Inertia of Rest: Is the inability of body to change the state of rest on its own.
Inertia of Motion: Is the inability of body to resist the change in state of motion on its own.
Inertia of direction: Is the inability of body to resist the change in direction of body on its own. Examples to understand more about Inertia

1. Have you noticed that passengers in a bus fall forward when bus stops suddenly.
REASON:- When the bus is moving, the bus and the passengers sitting in it are in motion. But when force is applied on the brakes, it stops, but due to inertia of motion the passengers remain in the state of motion, hence, fall forward.

2. Have you experienced that when carpet is beaten with a stick, the dust separates out
REASON:- Initially the carpet and dust are in state of rest. But when carpet is beaten with a stick, it comes in motion and moves backward. But due to inertia of rest, the dust particles remain in the state of rest hence, get separated.

3. Have noticed that leaves and fruits falls down when branch of tree is shaken vigorously
REASON:- Initially, the branch of a tree and its leaves and fruits are in state of rest. But when force is applied on the branch of a tree, the tree comes in motion but due to inertia of rest, the leaves and fruits remain in state of rest hence, falls down.

Exam Questions NCERT Class 9 Science Chapter 9 Force and Laws of Motion

Question. Mention the factors on which the force of friction depends?
Ans : (a) The force of friction is directly proportional to the weight of the body sliding over the surface.
(b) The force of friction depends on the surfaces in contact.

Question. What does first law of motion indicates when an unbalanced external force acts on an object?
Ans : First law of motion indicates that when an unbalanced external force acts on an object, its velocity changes i.e. the object gets acceleration.

Question. What happens to the velocity of an object when the force acting on the object are balanced?
Ans : An object can move with uniform velocity only when the forces (pushing force and friction force) acting on the object are balanced.

Question. What is resultant force?
Ans : The resultant force of several forces acting simultaneously on a body is that single force which produces the same effect on a body as all these forces produce together.

Question. How is inertia measured quantitatively?
Ans : Quantitatively the inertia of an object is measured by its mass.

Question. Why are roads on mountains inclined inwards at turns?
Ans : A vehicle moving on mountains is in the inertia of motion. At a sudden turn there is a tendency of vehicle to fall off the road due to sudden change in the line of motion hence the roads are inclined inwards so that the vehicle does not fall down the mountain.

Question. If a person jumps from a height on a concrete surface he gets hurt. Explain.
Ans :
When a person jumps from a height he is in state of inertia of motion. When he suddenly touches the ground he comes to rest in a very short time and hence the force exerted by the hard concrete surface on his body is very high, and the person gets hurt.

Question. When a force of 40 N is applied on a body it moves with an acceleration of 5 ms2. Calculate the mass of the body.
Ans :
Let m be the mass of the body.
Given : F = 40 N, a = 5 ms2
From the relation F = m a, we have
40 = m × 5
m= 40/5 = 8 kg

Question. Which will have more inertia a body of mass 10 kg or a body of mass 20 kg?
Ans : A body of mass 20 kg will have more inertia, as inertia is measure of mass.

Question. Name the factor on which the inertia of the body depends.
Ans : Inertia of a body depends upon the mass of the body.

Question. If a man jumps out from a boat, the boat moves backwards. Why?
Ans : When a man jumps out of a boat to the bank of the river, he supplies a force in the forward direction. Due to the reaction of this, the boat moves backwards.

Question. It is easier to push an empty box than to push the box full of books. Give reason.
Ans : This is because the empty box has less inertia of rest than the box full of books.

Question. Define SI unit of force.
                      or
What is one Newton force?
Ans : A Newton is that force which act on a body of mass of 1 kg to produce an acceleration of 1 ms^–2 in it.