Question

A motor car of mass 1200 kg is moving along a straight line with a uniform velocity of 90 $kmph$. Its velocity is slowed down to 18 $kmph$ in 4 seconds by an external force. The acceleration of the body is:

• A. $4m{s}^{-2}$
• B. $-4m{s}^{-2}$
• C. $-5m{s}^{-2}$
• D. $5m{s}^{-2}$

Answer 1

The correct option is C

$-5m{s}^{-2}$

Given, initial velocity $\left(u\right)=90kmph$ = $90\times \frac{5}{18}=25m{s}^{-1}$
Final velocity $\left(v\right)=18kmph$ = $18\times \frac{5}{18}=5m{s}^{-1}$
Mass of car = 1200 kg
Time (t) = 4 s

We know, acceleration $\left(a\right)=\frac{v-u}{t}$

So, $a=\frac{5-25}{4}=-5m{s}^{-2}$