Laws of Motion

Standard 9

Force

A push or pull that can cause an object to change its velocity or which can cause a

flexible object to deform.

Balanced Forces

Force F1 Force F2

F1 and F2 are acting on the rope. If both teams are evenly matched then F1 = F2. But F1 and F2 are in the opposite direction and hence cancel each other.The net force on the rope = 0.

When a number of forces act on a body such that the net force acting on the body is 0, then such a combination of forces are known as balanced forces.

Types of Forces

MuscularFrictional

Contact Forces

Non-Contact Forces

Magnetic Gravitational Electrostatic

Objects are Lazy – 1

Smooth tiled floor

Rough Ground

Football will remain at the same place unless someone kicks it.The football stops rolling after sometime as friction opposes its motion.The football travels more distance on a smooth surface as the frictional force is smaller.If there was no friction, the football would keep on rolling.

First the football does not want to move and once moving it does not want to stop (unless some force is applied).

Objects are Lazy – 2

Stack of books

Toy vehicle

Little guy

Wall

What will happen when the bus hits the wall?

The bus stops moving when it hits the wall. However the little guy on top of the bus continues moving and topples over.

Newton’s First Law of MotionAn object remains at rest or in motion in a straight line with a

constant speed until acted on by an unbalanced force.Tendency of an object to resist change is called Inertia.

Newton’s Second Law of MotionMore force is required to move or stop an object with more mass.

You need to kick harder if you want the football to travel at a greater speed / velocity.

You will have to pedal harder if you want to reach the top speed in less time.

𝑭𝒐𝒓𝒄𝒆∝𝑴𝒂𝒔𝒔∗𝑪𝒉𝒂𝒏𝒈𝒆 𝒊𝒏𝒗𝒆𝒍𝒐𝒄𝒊𝒕𝒚𝑻𝒊𝒎𝒆𝒕𝒂𝒌𝒆𝒏 ∝𝑴𝒂𝒔𝒔∗ 𝑨𝒄𝒄𝒆𝒍𝒆𝒓𝒂𝒕𝒊𝒐𝒏

Newton’s Second Law of MotionMomentum is defined as the total quantity of motion.

The rate of change of momentum is directly proportional to the impressed force and takes place in the direction in which the

force acts.

Newton’s Third Law of Motion

As the air escapes from the balloon it pushes the balloon forward.

Atmospheric pressure pushes the air out

Force of air on balloon pushes it forward

Wall

Straw String

Newton’s Third Law of Motion

Ball pushes the ground downwards

Ground pushes the ball upwards

When moving down, gravity acts in the direction of motion accelerating the ball.

When moving up, gravity opposes the motion decelerating the ball.

Newton’s Third Law of Motion

For every action there is an equal and opposite reaction.Action and Reaction act on different bodies and hence do not cancel each other.

When you take off your hand the force of the water pushes the block upwards.

The harder you push the block downwards, more the force with which water pushes the block back.

Block pushes the water downwards

Water pushes the block upwards

Conservation of Momentum

If there is no net force acting on two interacting bodies then their total momentum is conserved.

Units of MeasurementPhysical Quantity SI Units CGS Units

Force kg m/s2 or Newton g cm/s2 or dyne

Momentum kg m/s g cm/s