## Friday, August 19, 2011

### REVISION MATERIAL FOR CBSE CLASS 9 SCIENCE (PHYSICS) FOR SA 1

PHYSICS (MOTION)

Points to remember:

• A body is said to be at rest if it does not change its position with respect to its surroundings

• A body is said to be in motion i i changes its position with respect to surroundings

• we can perceive the motion of certain bodies directly such as a cycle moving on road.

• we can perceive the motion of certain other objects by indirect evidences.Motion of air can be observed from movement of dust particles and the branches of trees.

• rest and motion are relative terms.a body appearing to be at rest to one person may appear to be in motion of another person

• Speed is the ratio of distance covered to the time taken.speed is scalar quantity

• Speed is never negative and is zero when the body is at rest

• Velocity is the rate of change of displacement with time.It is a vector quantity

• Velocity may be zero,negative or positive even when tho body is moving. Its S I unit is m s -1

• Acceleration is the rate of change of velocity of a body.Acceleration is a vector quantity. Its  S I unit is m s -2.

• Motion . A body is said to be in motion  if it changes it position with respect to surroundings

• Uniform motion. Motion is said to be uniform if body covers equal distance in equal intervals of time however small these intervals may be.

• Non uniform motion.Motion is said to be nonuniform if the body covers unequal distances in equal intervals of time.

• Distance covered by body is the actual length of path covered by the body. It is measured in metre (in SI). Its CGS unit is centimetre (cm). Distance can never be negative. It is a scalar quantity.

• Displacement is the distance covered by a body in certain direction from starting point to final point. Displacement is a vector quantity and may be positive, negative or zero.

• Speed is the ratio of distance covered to the time taken