Acceleration of a body is the rate of change of its velocity with time. If v2 and v1 are instantaneous velocities at times t2 and t1 then the average acceleration is given by
= tan θ
The SI unit of acceleration is m/s2. On the v – t graph, the average acceleration is represented by the slope of the line joining the points (v1, t1) and (v2, t2) and the instantaneous acceleration at any point on the curve is represented by the slope of the tangent drawn at that point. Like velocity, acceleration is also a vector quantity with both magnitude and direction.
The acceleration of a body can change with either a change in the magnitude of its velocity or change in the direction of its velocity or both. Acceleration can be either positive or negative. The positive and the negative signs for acceleration depend on the direction of motion and the magnitude of velocity of the body. Zero acceleration stands for uniform velocity, i.e., the velocity does not change with time.