Acceleration and Deceleration (Retardation)

In the context of linear motion, the direction of the object in motion does not change; but the magnitude of the velocity may change.

When the velocity of an object changes, the object is said to be accelerating.

Acceleration is defined as the rate of change in velocity.

Acceleration is a vector quantity. SI unit for acceleration, a is ms^-2, metre per second squared.

Positive acceleration

If the velocity of an object increases from an initial velocity, u, to a higher final velocity, v.

The acceleration of an object is uniform if its velocity increases at a uniform rate.

Negative acceleration is called deceleration.

If the velocity of an object decreases from an initial velocity, u, to a lower final velocity, v, its acceleration is negative.

The deceleration of an object is uniform if its velocity decreases at a uniform rate.

Zero acceleration

An object moving at a constant velocity, that is, the magnitude and direction of its velocity remain unchanged - is not accelerating; its acceleration is zero.

Quiz :

An object accelerates at a constant rate from rest and reaches a velocity of 10 ms-1 after 5.0 s.

What is it acceleration?

Tips : rest mean stationary ( zero velocity )