Class 9 Science Chapter 9 Notes Force And Laws Of Motion

Class 9 Science Chapter 9 Notes Force And Laws Of Motion

CBSE Class 9 Science Chapter 9 Notes Force And Laws Of Motion on this step-by-step Force And Laws Of Motion answer guide . In some of State Boards and CBSE schools, students are taught thru NCERT books. As the chapter comes to an end, students are requested few questions in an exercising to evaluate their knowledge of the chapter. Students regularly want guidance dealing with those NCERT Class 9 Science Chapter 9 Notes Force And Laws Of Motion.

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Chapter 9 : Force And Laws Of Motion

Force: it is the force that enable us to do any work. to do anything, either we pull or push the object. Therefore, pull or push is called is force.


When we push or pull an object a force acts upon them and makes them move from one place to another.

  1. Initiate motion in a motionless object
  2. Change (increase or decrease) the velocity of the moving object
  3. Alter the direction of a moving object
  4. Change the shape and size of an object

Example, to open a door, either we push or pull it. A drawer is pulled to open and pushed to close.

Effect of Force

  • Force can move a stationary body or object . For example, a football can be set to move by kicking it, i.e., by applying a force.
  • Force can stop a moving body. For example, by applying brakes, a running cycle or a running vehicle can be stopped.
  • 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.
  • 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.
  • 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 mainly of two types:

  • Balanced forces
  • Unbalanced forces

( A ) 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 team apply similar magnitude of forces in opposite directions, the 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.

(B) Unbalanced Forces.

  1. 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 .

  1. Unbalanced forces can do the following:
  2. Move a stationary object
  3. Increase the speed of a moving object
  4. Decrease the speed of a moving object
  5. Stop a moving object
  6. Change the shape and size of an object

First Law of Motion

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 traveling 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.

Based on Galileo’s ideas Newton presented the three Laws of Motion:

First law of motion or The Law of Inertia

  • Whether an object is moving uniformly on a straight path or is at rest, its state would not change until and unless an external force is applied on to it.

Newton’s First Law of Motion (Law of Inertia):

Newton’s First Law of Motion: An object at rest will remain at rest unless acted on by an unbalanced force.

Explanation: If any object is in the state of rest, then it will remain in rest until an external force is applied to change its state. Similarly, an object will remain in motion until any external force is applied over it to change its state.

The state of any object can be changed by applying external forces only .

we can say that objects oppose a change in their state of motion or rest.


Tendency of objects to remain in the state of rest or to keep moving uniformly is called Inertia.

Examples of Inertia

  • We fall back when a vehicle starts moving in the forward direction because our body is in the rest state and it opposes the motion of the vehicle.
  • We fall forward when brakes are applied in a car because our body opposite the change of state of motion to rest.

Inertia and Mass

• The inertia of an object is dependent upon its mass.

• Lighter objects have less inertia, that is, they can easily change their state of rest or motion.

• Heavier objects have large inertia and therefore they show more resistance.

• Hence ‘ Mass ‘ is called a measure of the inertia of an object.

Example: It is easier for a person to push the bucket that is empty rather than the one that is filled with sand.

This is because the mass of an empty bucket is less than that of the bucket filled with sand.

The Second Law of Motion

  • Rate of the change of momentum of an object is proportional to applied unbalanced force in the direction of force
  • The impact produced by a moving object depends upon its mass and velocity.
  • For example, a small bullet fired at a high velocity can kill a person.
  • Momentum – The product of mass and velocity is called Momentum.
  • It is a vector quantity. Its direction is the same as that of the object’s velocity.
  • Denoted by – p
  • SI unit – kg meter per second
  • p = mv, (where m is the mass of the object, v is the velocity of the object.)
  • The momentum of a stationary object-
  • Let the mass of a stationary object be ‘m’,
  • Let the velocity of a stationary object be ‘v’,
  • The stationary object has no velocity, so v = 0,
  • Therefore, p = m*v = m*0 = 0

So, the momentum of a stationary object is zero.

  • We know that the velocity of an object can be changed by applying an unbalanced force on to it. Similarly, the momentum of an object can change by applying an unbalanced force.
  • According to the second law of motion­-

The rate of change of momentum of an object is directly proportional to the applied unbalanced force on the object in the direction of the force.

For Example –

A cricketer when catches a ball pulls his hands in the backward direction to give some time to decrease the velocity of the ball.

As the acceleration of the ball decreases the force exerted on catching the moving ball also decreases.

If the cricketer would try to stop a moving ball suddenly he would have to apply larger force.

Mathematical Formulation of the Second Law of Motion

Based on the definition of the second law of motion, we can infer that

Initial velocity = u

Final velocity = v

Acceleration = (v – u)/t

Change in momentum

p2 – p1

mv – mu

∝ m(v – u)

Rate of change of momentum ∝ m(v – u)/t

Force ∝ m(v – u)/t

Force = km(v – u)/t

Force = k ma

Force = ma

1 unit of force = k x (1 kg) X (1 ms-2)

K = 1

Therefore, with help of the second law of motion we can evaluate the amount of force that is being exerted on any object. From the formula started above, we can see that the force is directly propotional to acceleration. So the acceleration of an object can change depending upon the change in force applied.

Force = ma

SI Unit : kg-ms-2 or N (Newton)

The Third Law of Motion

Action and Reaction Forces

Two forces acting from opposite directions are called Action and Reaction Forces.

For example, a ball when hits the ground (action) bounces back with a certain force reaction.

The Third Law of Motion states that-

When an object exerts a force on another object, the second object instantly exerts a force back onto the first object.

These forces are always equal in magnitude but opposite in direction.

These forces act on two different objects always.

Or in other words, every action has an equal and opposite reaction.

The magnitudes of forces acting upon the objects are same but the acceleration produced in them may or may not be the same because the objects can differ in masses.

For Example, when a bullet is fired from a gun, the gun only moves a little backwards (recoils) while the bullet can travel a large distance. This is because of the difference in the mass of the bullet and the gun.

Conservation of Momentum

As per the law of conservation of momentum, the sum of momentum of two objects before the collision and after collision remains the same given that no external unbalanced force acts upon them.

In another way, collision conserves the total momentum of two objects.

Consider the figure given above. Two balls A and B having a certain initial velocities collide with each other.

Conditions before the collision-

• There is no unbalanced force acting upon them

• The initial velocity of A is greater than initial velocity of B.

The figure below explains how the momentum of the balls is conserved after the collision.

Facts about Conservation Laws

  • They are considered as the fundamental laws in physics.
  • They are based on observations and experiments.
  • They cannot be proved but can be verified or disproved with the help of experiments.
  • A single experiment is enough to disprove a law, while a single experiment is not enough to prove the same.
  • It requires a large number of experiments to prove the law.
  • The law of conservation of momentum was formulated 300 years ago.
  • There is no single situation present until now that disproves this law.
  • Other laws of conservation are – the law of conservation of energy, the law of conservation of angular momentum, the law of conservation of charge.

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