NCERT Exemplar Solutions Class 9 Science Force and Laws of Motion

Multiple Choice Questions (MCQs).................................

Question 1:  Which of the following statement is not correct for an object moving along a straight path in 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

Solution 1:   (c) It always goes away from the earth.

To travel away from the earth, the object's acceleration must be greater than gravity's acceleration. Option (c) is incorrect because moving along a straight path is insufficient to escape gravity.

Question 2:  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 direction
(d) act on either body at normal to each other

Solution 2:  (b) always act on different bodies in opposite directions. 

Action and reaction often operate in opposite directions on separate bodies, according to the third law of motion.

Question 3:  A goalkeeper in a game of football pulls his hands backwards after holding the ball shot at the goal. This enables the goalkeeper 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

Solution 3:   (d) decrease the rate of change of momentum.

After holding the ball, the goalkeeper pulls his hands backwards to slow down the rate of shift of momentum while increasing the time. Less force is exerted on his hands as a result of this (Force is directly proportional to the rate of change of momentum).

Question 4:  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

Solution 4:  (c) to resist any change in its state of motion.

An object's inertia causes the object to resist any change in its state of rest or motion.

Question 5: 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

Solution 5:  (a) accelerated. 

If the coin lands behind the passenger that means the train is speeding up. As the coin is thrown, it has the same velocity as the train, but as it travels through the air, its velocity decreases (due to the train's acceleration), and it falls behind the passenger.

Question 6:  An object of mass 2 kg is sliding with a constant velocity of 4 ms^-1 on a frictionless horizontal table. The force required to keep the object moving with the same velocity is
(a) 32 N                   

(b) 0 N                      

(c) 2 N                     

(d) 8 N

Solution 6:  (b) 0 N. 

Given, mass m = 2 kg, velocity v = 4 ms^-1
As the object is moving with a constant velocity i.e., 4 ms^-1 so the acceleration of the object is zero i.e., a = 0 and Whether there is no external force acting on the body, according to the property of inertia, the body remains as it is, i.e., if the body is at rest, it remains at rest, and if it is in motion, it remains in motion.

Question 7:  Rocket works on the principle of conservation of

(a) mass                  

(b) energy                

(c) momentum        

(d) velocity

Solution 7:   (c) momentum.  

Rocket is based on the theory of momentum conservation. The fuel in a rocket burns and emits gas at a high temperature. These gases are expelled from the rocket through a nozzle on the rocket's back side. The rocket accelerates due to the forward force exerted by the ejecting gas.
Due to their immense velocity of escape, the mass of gases escaping per second is very small, but their momentum is very high. The rocket receives an equal and opposite momentum, allowing it to achieve a high velocity despite its large mass.

Question 8:  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 

Solution 8:  (b) move forward. 

The tanker will come to a halt when the brake is applied abruptly, but the water will continue to move forward because it is already flowing.

Short Answer Type Questions.........................

Question 9:  There are three solids made up of aluminium, steel and wood of the same shape and same volume. Which of them would have highest inertia?

Solution 9:   Since mass is a measure of inertia, a ball with the same shape and size but more mass would have the highest inertia. Since steel has the highest density and mass, it also has the highest inertia.

Question 10:  Two balls of the same size but of different materials, rubber and iron, are kept on the smooth floor of a moving train. The brakes are applied suddenly to stop the train. Will the balls start rolling? If so, in which direction? Will they move with the same speed? Give reasons for your answer.

Solution 10:   The train comes to a full halt when it comes to a complete stop, but the balls stay in motion. As a result of the inertia of motion, the balls roll forward. Since the balls are the same size but made of different materials, their mass may vary. As a result, the two balls will pass at different rates.

Question 11:  Two identical bullets are fired one by one by a light rifle and another by a heavy rifle with the same force, which rifle will hurt the shoulder more and why?

Solution 11:    Since both bullets are similar and shot with the same intensity, they are indistinguishable. So, in order to minimize recoil velocity, we use a heavy rifle with a large mass to combat the large momentum. However, because of the small mass of a light weapon, the recoil velocity would be high, causing more shoulder pain.

Question 12:  A horse continues to apply a force in order to move the cart with a constant speed. Explain why?

Solution 12:   As the cart begins to travel, frictional force acts on the cart's wheel in the opposite direction of the motion. To maintain constant speed, the horse must apply continuous force in the forward direction.

Question 13:  Suppose a ball of mass m is thrown vertically upward with an initial speed v, its speed decreases continuously till it becomes zero. Thereafter, the ball begins to fall downward and attain the speed v again before striking the ground. It implies that the magnitudes of initial and final momentums of the ball are same. Yet, it is not an example of conservation of momentum. Explain why?

Solution 13:  If no external force acts on a system, the momentum remains constant. Since the ball is subjected to an external force, gravitational force, in the given case, it is not an example of conservation of momentum.

Question 14:  Velocity versus time graph of a ball of mass 50 g rolling on a concrete floor is shown in figure. Calculate the acceleration and frictional force of the floor on the ball.