A body is moving vertically upwards. Its velocity changes at a constant rate from 50 m s ^-1 to 20 m s ^-1 in 3 s . What is its acceleration ?

As we know,

$\text {a} = \dfrac {\text {final vel. (v)} - \text {initial vel. (u)}}{\text {time (t)}}$

Given,

Initial velocity (u) = 50 m s ^-1

Final velocity (v) = 20 m s ^-1

Time (t) = 3 s

Substituting the values in the formula above, we get,

$\text {Acceleration (a)} = \dfrac {20 - 50}{3} \\[0.5em] \text {Acceleration (a)} = - \dfrac {30\text { ms}^{-1}}{3\text { s}} \\[0.5em] \Rightarrow \text {Acceleration (a)} = -10 \text { ms}^{-2} \\[0.5em]$

Hence, acceleration of the body is -10 m s ^-2. Negative sign shows that the velocity decreases with time, so retardation is 10 m s ^-2.