# 9th Class Physics Chapter 3 Notes

A force is a push or pull. It moves or tends to move, stops or tends to stop the motion of a body.

Inertia of a body is its property due to which it resists any change in its state of rest or uniform motion in a straight line.

Momentum of a body is the quantity of motion possessed by the body. Momentum of a body is equal to the product of its mass and velocity

The force that opposes the motion of a body is called friction.

Newton’s first law of motion states that a body continues its state of rest or of uniform motion in a straight line provided no net force acts on it.

Newton’s second law of motion states that when a net force acts on a body, it produces acceleration in the body in the direction of the net force. The magnitude of this acceleration is directly proportional to the net force acting on it and inversely proportional to its mass. Mathematically, F = ma

SI unit of force is newton (N). It is defined as the force which produces an acceleration of 1 ms-2 in a body of mass 1 kg. moves on a smooth horizontal surface are given by:

a = m1 g ; T = m1 m2 g

m1+m2 m1+m2

Law of conservation of momentum states that the momentum of an isolated system of two or more than two interacting bodies remains constant.

A force between the sliding objects which opposes the relative motion between them is called friction.

Rolling friction is the force of friction between a rolling body and a surface over which it rolls. Rolling friction is lesser than the sliding friction.

The friction causes loss of energy in machines and much work has to be done in overcoming it. Moreover, friction leads to much wear and tear on the moving

Mass of a body is the quantity of matter possessed by it. It is a scalar quantity. SI unit of mass is kilogramme (kg).

Weight of a body is the force of gravity acting on it. It is a vector quantity. SI unit of weight is newton (N).

Newton’s third law of motion states that to every action there is always an equal and opposite reaction.

The acceleration and tension in a system of two bodies attached to the ends of a string that passes over a frictionless pulley such that both move vertically are given by:

a = m1 – m2 g ; T = 2m1 m2 g

m1+m2 m1+m2

The acceleration and tension in a system of two bodies attached to the ends of a string that passes over a frictionless pulley such that one moves vertically and the other parts of the machine. The friction can be reduced by:

Smoothing the sliding surfaces in contact.

Using lubricants between sliding surfaces.

Using ball bearings or roller bearings.

The motion of a body moving along a circular path is called circular motion.

The force which keeps the body to move in a circular path is called the centripetal force and is given.Fc = mv2/r

According to Newton’s third law of motion, there exists a reaction to the centripetal force. Centripetal reaction that pulls the string outward is sometimes called the centrifugal force.