A car is moving with a velocity 20 m s ^-1 . The brakes are applied to retard it at a rate of 2 m s ^-2. What will be the velocity after 5 s of applying the brakes ?

As we know,

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

and retardation is negative acceleration.

Hence,

$\text {final vel. (v)} - \text {initial vel. (u)} \\[0.5em] = \text {Retardation(-a)} \times {\text {time(t)}}$

Given,

a = - 2 m s^-2

t = 5 s

u = 20 m s ^-1

Substituting the values in the formula above, we get,

$\text {final vel. (v)} - 20 \text{m s}^{-1} = - 2 \text{m s}^{-2} \times 5 s \\[0.5em] \text {v} - 20 \text{m s}^{-1} = - 10 \text{m s}^{-1} \\[0.5em] \Rightarrow \text {v} = (20 - 10) \text{m s}^{-1} \\[0.5em] \Rightarrow \text {v} = 10 \text{m s}^{-1} \\[0.5em]$

Hence, final velocity = 10 m s ^-1.