Force And Laws Of Motion Class 9 Notes Science

 Force And Laws Of Motion Class 9 Notes CBSE Science Chapter 9 

The CBSE NCERT Based Class 9th Revision Notes for Science Chapter 9, Force and Laws of Motion. These notes serve as a comprehensive review of the key concepts and topics presented in the NCERT TextBooks for Class 9th Science. The Chapter Notes are organized in a topic/concept-wise manner, providing detailed explanations for each topic covered in the textbook, including but not limited to, the Introduction to forces and free body diagrams, Newton's laws of motion (first, second, and third), Normal force and contact force, Balanced and unbalanced forces, Inclined planes motion, Distance and Displacement, Speed and Velocity, and Acceleration.

Well prepared Class 9th Science all Chapters Notes will guidance students understand the topics ,concepts and themes covered in all the chapters of Ncert Text book of 9th  

Force And Laws Of Motion

Force

It is the force that enables us to do any work. To do anything, either we pull or push the object.

Therefore, pull or push is called force.

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 make a stationary body in motion.

Example, a football can be set to move by kicking it i.e. by applying a force.

• Force can stop a moving body.

Example, by applying brakes, a running cycle or a running vehicle can be stopped.

• Force can change the direction of a moving object.

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.

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 :

(i) Balanced forces

(ii) 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 team 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.

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.

• Force can change the direction of a moving object.

➢ 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

Newton's Laws of Motion

• Galileo Galilei: Galileo first of all said that object move with a constant speed when no foces act 

on them.

• This means if an object is moving on a frictionless path and no other force is acting upon it, the 

object would be moving forever. That is, there is no unbalanced force working on the object.

• But practically it is not possible for any object. Because to attain the condition of zero, 

unbalanced force is impossible.

• Force of friction, force of air and many other forces are always acting upon an object.

• But practically it is not possible for any object. Because to attain the condition of zero, 

unbalanced force is impossible.

Newton's Laws of Motion

• Newton studied the ideas of Galileo and gave the three laws of motion. These laws are known as 

Newton's laws of motion.

(i) First Law of Motion

(ii) Second Law of Motion

(ii) Third Law of Motion

Newton's First Law of Motion (Law of Inertia)

• 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 until a 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.

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

(i) A person standing in a bus falls backward when bus starts 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 the tendency to remain in 

rest. Because of this, the person falls backward; if he is not alert.

(ii) A person standing in a moving bus falls forward if driver applies 

breaks 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 contact with the bus come in rest while the rest 

part of his body have the tendency to remain in motion. Because of this person falls froward if he 

is not alert. 

(iii) Before hanging the wet clothes over laundry line, usually many jerks are given to the clothes

to get them dried quickly. Because of jerks, droplets of water from the pores of the cloth falls on

the ground and reduced amount of water in clothes dries them quickly.

• This happens because when suddenly clothes are made in motion by giving jerks, the water

droplets in it have the tendency to remain in rest and they are separated from clothes and fall on

the ground.

(iv) When the pile of coin on the carom-board is hit by a striker,

coin only at the bottom moves away leaving rest of the pile of

coin at same place.

• This happens because when the pile is struck with a striker, the

coin at the bottom comes in motion while rest of the coin in the pile has the tendency to remain in

the rest and they vertically falls the carom-board and remain at same place.

Mass and Inertia

• The property of an object because of which it resists to get disturb 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 the 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 pull a heavy box over the

ground than the lighter one.

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 × Velocity (p = m × v) where, p = momentum,

m = mass of the object and v = velocity of the object.

• Some explanations to understand the momentum:

• A person get injured in the case of hitting by a moving object, such as stone, pebbles or anything

because of momentum of the object.

• Even a small bullet is able to kill a person when it is fired from a gun because of its momentum

due to great velocity.

• A person get injured severely when hit by a moving vehicle because of momentum of vehicle due

to mass and velocity.

Momentum and Mass

• Since momentum is the product of mass and velocity (p = m × v) of an object. This means

momentum is directly proportional to mass and velocity. Momentum increases with increase of

either mass or velocity of an object.

• This means if a lighter and a heavier object is moving with same velocity, then heavier object will

have more momentum than the lighter one.

• If a small object is moving with great velocity, it has tremendous momentum. And because of

momentum, it can harm an object more severely.

Example : a small bullet having a little mass even kills a person when it is fired from a gun.

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