# Motion Class 9 Notes Science - Chapter 8

## Motion: Moving Of Body/object

Newton’s First Law of Mot ion

Body continues its state of rest or of uniform motion in a straight line unless an external force acts on it.

Inertia: Property of the body by virtue of which it opposes any change in its state of rest or of uniform motion in a straight line.

• Measure of inertia: Mass
• Greater mass have greater inertia
 Inertia of Rest: A body at rest tends to remain at rest. Inertia of Motion: A body in uniform motion tends to remain in its motion. Eg: When a bus suddenly starts moving forward, passengers in the bus fall backward. Eg: Passengers fall forward when a fast moving train stops suddenly.

Newton’s Second Law of Mot ion

Rate of change of momentum is directly proportional to applied force and change in momentum is along the direction of applied force.

F ∝ (ΔP/Δt)   or F = (ΔP /Δt)

Newton’s Third Law of Mot ion

To every action there is an equal and opposite reaction.

$$\overrightarrow{F_{12}}=\overrightarrow{F_{21}}$$

• In walking, the feet push the ground and in tern ground exerts equal force in opposite direction.

## Force

It is a kind of push or pull to change the state of body/object.

## Force

Unit of Force

• Newton (SI)
• Dyne (Cgs)

Types of Force

• Balanced: Two or more forces acting on an object or a body if the net effect produced by them is zero
• Unbalanced: Two or more forces acting on an object or a body it the net effect produced by them is not zero

## Momentum

• Product of mass and velocity, P = mv
• SI unit: Kg m/s

Conservation of Momentum

If external forces acting on a system of bodies is zero, the net momentum remains conserved.