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Forces and Laws of Motion- Class 9 Science Notes


Force can be called as a push or pull on the object that enables us to do any work.

For example in order to open the door we either push or pull it. Similarly, when a drawer is polled or pushed to open or close it forces said to have been applied.

Effects of force

• It can make a stationary body move

• In stop a moving body

• It can change the direction of a moving object

• It can change the speed of a moving body

• Change the shape and size of an object

Forces are mainly of two types:

A) Balanced forces

B) Unbalanced forces

Balanced forces

• The resultant of applied forces is equal to 0

• It does not cause any change of state of an object since the forces are equal in magnitude and opposite in direction

• It can change the shape and size of the object

Un-Balanced forces

• The resultant of applied forces is greater than zero

• Unbalance forces can move a stationary object, increase or decrease the speed of a moving object, stop a moving object, change the shape and size of the object

Laws of motion:

Galileo Galilie said that when an object is moving on a frictionless surface and no other force is acting upon it, then the object would move forever. To say, there is no unbalanced force working on the object.

But in real life it is not possible to attain the condition of zero unbalanced forces. Forces such as force of friction, force of air and many other forces are always acting upon an object.

Newtons laws of motion:

Newton studied the ideas of Galileo and gave the three laws of motion. These laws are now

known as newtons laws of motion

Newtons first law of motion or law of inertia: an object remains in the state of rest or uniform motion along a straight line, until it is compelled to change the state by application of an external force.

In simple words, if an object is in state of rest then it will remain in rest until an external

unbalanced force is applied on it. Similarly, an object in motion will remain in motion until an

external unbalanced force is applied to change its state.

This means that objects resist the change in the state and these changes can only be brought by applying external unbalanced forces.

Inertia: It can be called the property of an object because of which it resists change. Inertia of an object is measured by its mass and is directly proportional to it.

This means that with the increase in mass the inertia increases and vice versa. I have your object will have greater Inertia than a lighter one.

It is because of inertia that a heavy object is difficult to push or pull.

Examples of newtons first law of motion in every day life:

A) A person standing in a bus falls backwards when the bus starts moving because initially the person in the bus were both at rest but when the bus starts moving the legs of the person starts moving along with it but the rest of his body tends to be at rest. And hence the person falls backward if they are not alert.

B) Wet clothes are given a jerk to get them dried quickly as the droplets of water from the wet cloth falls on the ground allowing the clothes to dry faster. It happens because then water droplets has a tendency to remain addressed after the joke to the piece of clothes and are separated.

C) A fast moving striker only moves the coin from the bottom of the pile while the rest of the file remains at the same place. This also happens because the rest of the coins have the tendency to remain in rest.

What is momentum?

• Momentum can be described as the intensity of motion of an object. It is mathematically

defined as a product of velocity and mass of the object and is denoted by ‘p’

Momentum of object = mass x velocity

p = m x v

SI unit of momentum = kgm/s

A small bullet is able to kill a person because it has a very high momentum by virtue of its velocity even though it has a small mass.

Since momentum is the product of mass and velocity of an object, It is directly proportional to mass in velocity and hence it increases with increase in either mass or velocity or both.

If a lighter and heavier objects are moving with the same velocity then the heavier object will have greater momentum.

If a small Object is moving with great velocity then it has a very high momentum and because of this high momentum it can harm an object severely as in the case of a bullet.

Newtons second 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.


mass of an object = m kg

Initial velocity of the object = u m/s

Final velocity of the object = v m/s

Initial momentum = mu and Final momentum = mv


Change in momentum = Final momentum - Initial Momentum

= mv -mu

= m(v-u)

Rate of change of momentum = Change in momentum/ Time taken

But according to the second law of motion this rate of change of momentum is directly

proportional to force.

Define 1 N.

When an acceleration of 1 m/s2 is seen in a body of mass 1 kg, then the force applied on the body is said to be 1 Newton.

Proof of newtons first law of motion from second law

The first law states that if external force is equal to 0 then a moving body keeps moving with the same velocity or if the body is at rest then it continues to be at rest.

Newtons third law of motion

To every action there is an equal and opposite reaction. These forces act on two different bodies

Applications of the third law of motion:

• Walking and Running

• Rowing of a boat

• Recoiling of a gun

Law of conservation of momentum

When two or more bodies active on one another, the total momentum remains constant provided no external forces are acting on it.

Initial momentum = Final momentum

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