FORCE & LAWS OF MOTION

IX PHYSICS

YESH CHOPRA

IX- B

11

SCIENCE FA

FORCE & LAWS OF MOTION

FORCE

• In physics, a force is any influence that causes a free body to undergo a change in speed, a change in direction, or a change in shape.

• Force is a vector quantity• It is the cause of acceleration and retardation

in a object

• F=ma• It’s unit is Newton

Generally pull and push is known as force.

Force can make a stationary body in motion

For example a football can be set to move

by kicking it

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.

Force can stop a moving body

For example by applying brakes,

a running cycle or a running vehicle

can be stopped.

Effect of Force

Force can change the speed of a moving body

A hockey player hits the the ball

coming towards him/her and

in this way the speed of the

ball can be changed.

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.

Effect of Force

Types of Force

There are 2 main types of forces:

1. Balanced Force- Which is cause of rest.

2. Unbalanced Force Which is cause of Motion.

Inertia

• The property of an object by virtue of which it is unable to change its rest state or uniform motion state itself, is known as Inertia.

• Inertia is the natural tendency of an object to resist a change in its state of motion or of rest. The mass of an object is a measure of its inertia.

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. Intertia 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.

Mass and InertiaInertia of an object is directly proportional to its mass

• Heavy object has large inertia.

• Light object has less inertia.

Laws of motion

Galileo’s observation•By observing the motion of objects on an inclined plane Galileo deduced that objects move with a constant speed when no force acts on them.• He observed that when a marble rolls down an inclined plane, its velocity increases.

VS

At the end of comparison, Aristotle’s laws were failed and hence ,Galileo proved his laws wrong and won.

Newton’s Laws of motion

1. Law of Inertia 2. Law of momentum3. Law of action and reaction

Sir Isaac Newton made a systematic study of motion and proposed three law of motion witch are known as Newton's laws of motion.

Law of InertiaLaw of inertia on rest

• If an object is in rest , it will remain in the rest untill and unless an external force is not applied on it.

Law of inertia on motion

• If an object is in uniform motion in a straight path , it will remain in uniform motion , untill and unless an external force is not applied on it.

1st law of motion from 2nd law

That is, when F = 0, v = u for whatever time, t is taken. This means that the object will continue moving with uniform velocity, u throughout thetime, t. If u is zero then v will also

be zero.That is, the object will remain at rest.

• Dust is removed from a hanging carpet by beating it with a stick.

• Coin falls into the tumbler when the card is given a sudden jerk.

• Only the carom coin at the bottom of a pile is removed when a fast moving carom striker hits it.

• An athlete runs for a certain distance before taking a long jump.

• A ball thrown upward in a moving train comes back into the thrower’s hands.

Law of Momentum

• The rate of change of linear momentum is equal to the force applied and the direction of force is always in the direction of change in momentum.

MATHEMATICAL FORMULATION OFSECOND LAW OF MOTION

Every action has its equal and opposite reaction.

Law of Action & Reaction

• While walking ,we press the ground (action) in backward direction ,it pushes us in forward direction with equal force but in the opposite direction.

• The Rocket is launching in the backward direction and flies in forward direction.

The recoiling of gun also due to action and reaction.

A skateboard surfer exerts his action and in reaction the skate board runs

Conservation of Momentum

According to II law of motion

F = 𝑑𝑃

𝑑𝑡

If external applied force F in zero (F=0)

Then F = 𝑑𝑃

𝑑𝑡=0

i.e. P = Constant

In the absence of ext. applied force net linear momentum of a system is always constant.

Collision of two ballsAccording to the third law of motion, the force F exerted by ball A on ball B (action) and the force F exerted by the ball B on ball A (reaction) must be equal and opposite to each other.

So, the total momentum of the two balls remains unchanged or conserved provided no other external force acts.