Study Notes for CBSE Class 9 Science Chapter 9 Force and Laws of Motion

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Study Notes for CBSE Class 9 Science Chapter 9 Force and Laws of Motion

Forces 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.

Effects of Force
(i) Force can make a stationary body in object. For example, a football can be set to move by kicking it, i.e., by applying a force.

(ii) Force can stop a moving body. For example, by applying brakes, a running cycle or a running vehicle can be stopped.

(iii) Force can change the direction of a moving object.

(iv) 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.

(v) 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 :

(A) Balanced forces

(B) 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, 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
• 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 :

Galileo Galilei : Galileo first of all said that object move with a constant speed when no forces 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.

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.

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 :

(a) 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.

(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 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 forward if he is not alert.

(c) 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.

(d) 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.


• Momentum is the power of motion of an object.
• The product of velocity and mass is called the momentum. Momentum is denoted by ‘p’.

Momentum and Mass and Velocity

• 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. For example, a small bullet having a little mass even kills a person when it is fired from a gun.

• Usually, road accidents prove more fatal because of high speed than in slower speed. This happens because vehicles running with high speed have greater momentum compared to a vehicle running with slower speed.

Units of Momentum:

SI unit of mass = kg
SI unit of velocity = meter per second i.e., m/s

Therefore, SI unit of momentum = kg m/s

Statement of Second Law

Rate of change of momentum of an object is proportional to applied unbalanced force in the direction of force.

Third Law of Motion

To every action there is an equal an opposite reaction.

Applications :

(i) Walking is enabled by IIIrd law.

(ii) A boat moves back when we deboard it.

(iii) A gun recoils.

(iv) Rowing of a boat.

Law of Conservation of Momentum

When two (or more) bodies act upon one another, their total momentum remains constant (or conserved) provided no external forces are acting.

Initial momentum = Final momentum

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What are the advantages of friction?

Due to friction we are able to walk on the surface of the Earth, The brakes applied in automobiles work only due to friction, Nails, screws and the wooden boards are held together due to force of friction, The fibres of thread are held together due to force of friction.

What are the disadvantages of force of friction?

In overcoming the friction, a lot of energy is wasted in the form of heat. Friction causes wear and tear of the moving parts

What is Inertia?

Inertia can be defined as the property of matter by virtue of which it opposes any change in its state of rest or of uniform motion along a straight line.

Can unbalanced force change the direction of an object?

Yes, an unbalanced force can change the speed or direction or both of an object.

What is impulse?

Impulse is a large force acting on an object for a very short duration. Tt is defined as the product of force and time for which it acts on the object.

What are examples of impulsive force?

When we kick a football, the kick lasts only for a fraction of a second. The force, which we apply on a football, is an example for impulsive force.

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