CBSE Class 9 Science Force and laws of motion Notes

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FORCE

A force is anything that can cause a change to objects. Forces can:

• change the shape of an object

• move or stop an object

• change the direction of a moving object.

A force can be classified as either a contact force or a non-contact force.

A contact force must touch or be in contact with an object to cause a change. Examples of contact forces are:

• the force that is used to push or pull things, like on a door to open or close it

• the force that a sculptor uses to turn clay into a pot

• the force of the wind to turn a windmill

A non-contact force does not have to touch an object to cause a change. Examples of noncontact forces are:

• the force due to gravity, like the Earth pulling the Moon towards itself

• the force due to electricity, like a proton and an electron attracting each other

• the force due to magnetism, like a magnet pulling a paper clip towards itself

The unit of force is the newton (symbol N). This unit is named after Sir Isaac Newton who first defined force. Force is a vector quantity and has a magnitude and a direction.

EFFECT OF FORCE:

1. Force can make a stationary body in motion. For example a football can be set to move by kicking it, i.e. by applying a force.

2. Force can stop a moving body – For example by applying brakes, a running cycle or a running vehicle can be stopped.

3. Force can change the direction of a moving object. For example; By applying force, i.e. by moving handle the direction of a running bicycle can be changed. Similarly by moving steering the direction of a running vehicle is changed.

4. Force can change the speed of a moving body – By accelerating, the speed of a running vehicle can be increased or by applying brakes the speed of a running vehicle can be decreased.

5. Force can change the shape and size of an object. For example -– By hammering, a block of metal can be turned into a thin sheet. By hammering a stone can be broken into pieces.

Forces are can also divided into two types:

1. Balanced Forces

2. Unbalanced Forces

BALANCED FORCES

If the resultant of applied forces is equal to zero, it is called balanced forces.
Example : - In the tug of war if both the teams apply similar magnitude of forces in opposite directions, rope does not move in either side. This happens because of balanced forces in which resultant of applied forces become zero.
Balanced forces do not cause any change of state of an object. Balanced forces are equal in magnitude and opposite in direction.
Balanced forces can change the shape and size of an object. For example - When forces are applied from both sides over a balloon, the size and shape of balloon is changed.

UNBALANCED FORCES

If the resultant of applied forces are greater than zero the forces are called unbalanced forces.
An object in rest can be moved because of applying balanced forces.
Unbalanced forces can do the following:
• Move a stationary object.
• Increase the speed of a moving object.
• Decrease the speed of a moving object.
• Stop a moving object.
• Change the shape and size of an object.

LAWS OF MOTION:

NEWTON’S LAWS OF MOTION:

• Newton's First Law of Motion - Any object remains in the state of rest or in uniform motion along a straight line, until it is compelled to change the state by applying external force.
• Newton's Second Law of Motion - The rate of change of momentum is directly proportional to the force applied in the direction of force.
• Newton's Third Law of Motion - There is an equal and opposite reaction for evrey action

NEWTON’S FIRST LAW OF MOTION:

Any object remains in the state of rest or in uniform motion along a straight line, until it is compelled to change the state by applying external force.
Explanation: If any object is in the state of rest, then it will remain in rest untill a exernal force is applied to change its state. Similarly an object will remain in motion untill any external force is applied over it to change its state. This means all objects resist to in changing their state. The state of any object can be changed by applying external forces only.

NEWTON’S FIRST LAW OF MOTION IN EVERYDAY LIFE:

a. A person standing in a bus falls backward when bus is start moving suddenly. This happens because the person and bus both are in rest while bus is not moving, but as the bus starts moving the legs of the person start moving along with bus but rest portion of his body has tendency to remain in rest. Because of this person falls backward; if he is not alert.
b. A person standing in a moving bus falls forward if driver applies brakes suddenly. This happens because when bus is moving, the person standing in it is also in motion along with
bus. But when driver applies brakes the speed of bus decreases suddenly or bus comes in the state of rest suddenly, in this condition the legs of the person which are in the contact
with bus come in rest while the rest parts of his body have tendency to remain in motion. Because of this person falls forward if he is not alert.

MASS AND INERTIA:

The property of an object because of which it resists to get disturbed its state is called Inertia.
Inertia of an object is measured by its mass. Inertia is directly proportional to the mass. This means inertia increases with increase in mass and decreases with decrease in mass. A heavy object will have more inertia than lighter one.
In other words, the natural tendency of an object that resists the change in state of motion or rest of the object is called inertia.
Since a heavy object has more inertia, thus it is difficult to push or pul a heavy box over the ground than lighter one.

INTEXT QUESTIONS PAGE NO. 118

1. 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-rupees coin and a one-rupee coin?

Ans. Inertia is the measure of the mass of the body. The greater is the mass of the body; the greater is its inertia and vice-versa.
(a) Mass of a stone is more than the mass of a rubber ball for the same size. Hence, inertia of the stone is greater than that of a rubber ball.
(b) Mass of a train is more than the mass of a bicycle. Hence, inertia of the train is greater than that of the bicycle.
(c) Mass of a five rupee coin is more than that of a one-rupee coin. Hence, inertia of the five rupee coin is greater than that of the one-rupee coin.

2. 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.

Ans. The velocity of the ball changes four times. As a football player kicks the football, its speed changes from zero to a certain value. As a result, the velocity of the ball gets changed. In this case, the player applied a force to change the velocity of the ball. Another player kicks the ball towards the goal post. As a result, the direction of the ball gets changed. Therefore, its velocity also changes. In this case, the player applied a force to change the velocity of the ball. The goalkeeper collects the ball. In other words, the ball comes to rest. Thus, its speed reduces to zero from a certain value. The velocity of the ball has changed. In this case, the goalkeeper applied an opposite force to stop/change the velocity of the ball. The goalkeeper kicks the ball towards his team players. Hence, the speed of the ball increases from zero to a certain value. Hence, its velocity changes once again. In this case, the goalkeeper applied a force to change the velocity of the ball.

3. Explain why some of the leaves may get detached from a tree if we vigorously shake its branch.

Ans. Some leaves of a tree get detached when we shake its branches vigorously. This is because when the branches of a tree are shaken, it moves to and fro, but its leaves tend to
remain at rest. This is because the inertia of the leaves tend to resist the to and fro motion. Due to this reason, the leaves fall down from the tree when shaken vigorously.

4. Why do you fall in the forward direction when a moving bus brakes to a stop and fall backwards when it accelerates from rest?

Ans. Due to the inertia of the passenger. Every body tries to maintain its state of motion or state of rest. If a body is at rest, then it tries to remain at rest. If a body is moving, then it tries to remain in motion. In a moving bus, a passenger moves with the bus. As the driver applies brakes, the bus comes to rest. But, the passenger tries to maintain his state of
motion. As a result, a forward force is exerted on him. Similarly, the passenger tends to fall backwards when the bus accelerates from rest. This is because when the bus accelerates,
the inertia of the passenger tends to oppose the forward motion of the bus. Hence, the passenger tends to fall backwards when the bus accelerates forward.

MOMENTUM

Momentum is the power of motion of an object.
The product of velocity and mass is called the momentum. Momentum is denoted by ‘p’.
Therefore, momentum of the object = Mass x Velocity.
Or, p = m x v
Where, p = momentum, m = mass of the object and v = velocity of the object.

NEWTON'S SECOND LAW OF MOTION

Newton's second Law of Motion states that The rate of change of momentum is directly proportional to the force applied in the direction of force.
For example; when acceleration is applied on a moving vehicle, the momentum of the vehicle increases and the increase is in the direction of motion because the force is being applied in the direction of motion. On the other hand, when brake is applied on the moving vehicle, the momentum of the vehicle decreases and the decrease is in the opposite direction of motion because the force is being applied in the opposite direction of motion.

By substituting the value of ‘k = 1’ in equation (ii) we get
F = m.a ……………… (iii)
⇒ Force = mass × acceleration
Thus Newton’s Second Law of Motion gives the relation between force, mass and acceleration
of an object.
According to the relation obtained above, Newton’s Second Law can be modified as follows:
The product of mass and acceleration is the force acting on the object.
The SI unit of Force: Newton (N)
Since Force = Mass x Acceleration
The unit of mass = kg and The unit of acceleration = m/s2
If force, mass and acceleration is taken as 1 unit.
Therefore,
1 Newton (N) = 1kg x 1m/s2
Thus, Newton (N) = kg m/s2
Equation (iii) can be also written as
a = F/m ………………… (iv)
This equation is the form of Newton’s Second Law of Motion.
According to this equation, Newton’s Second Law of Motion can also be stated as follow:
The acceleration produced by a moving body is directly proportional to the force applied over
it and inversely proportional to the mass of the object.
From the above relation it is clear that
Acceleration increases with increase in force and vice versa.
Acceleration decreases with increase in mass and vice versa.
That’s why a small vehicle requires less force to attain more acceleration while a heavy vehicle requires more force to get the same acceleration

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