# what is Acceleration

## Force and pressure of Class 8

Mostly the velocity of a moving object changes either in magnitude or in direction or both when the object moves. The body is then said to have acceleration. So it is the rate of change of velocity i.e. `change in velocity per unit time is said to be acceleration ( it is a vector quantity).

• Static Friction: The opposite force that comes into play when one body tends to move over the surface of another, but the actual motion has yet not started is called static friction.
• If the applied force is P and the body remains at rest then static friction F = P.
• If a body is at rest and no pulling force is acting on it, the force of friction on it is zero.

### UNITS OF FORCE:

∴ F = ma → gram × cm/s2 = dyne

If m = 1 gram, a = 1 cm/s2, then F = 1 dyne

When a force is applied on a 1 gram body and the acceleration produced in the body is 1 cm/s2 then the force acting on the body will be one dyne.

• In C.G.S. system:
• In S.I. system :

F = ma → kg × m/s2 = Newton

If m = 1 kg and a = 1 m/s2 then by F = ma,

F = 1 × 1 = 1 kg x m/s2 = 1 Newton.

If a force is applied on a body of mass 1 kg and acceleration produced in the body is 1 m/s2 then the force acting on the body will be one Newton.