Question

A car weighing 600 kg traveling with $72km/h$ stops at a distance of $60m$ decelerating uniformly. What is the work done by brakes in joules?

• A. $1.2\times {10}^{5}J$
  
• B. $2.4\times {10}^{5}J$
• C. $3.2\times {10}^{5}J$
• D. $1.4\times {10}^{5}J$

Answer 1

Answer: (A)

$1.2\times {10}^{5}J$

Given, initial velocity $u=72km/h=\frac{72\times 1000}{3600}m/s=20m/s$; final velocity $v=0m/s$ and displacement $S=60m$

Using formula, $v^2=u^2-2aS\Rightarrow 0^2={20}^2-2a\left(60\right)$

or $a=400/120=10/3m/s^2$

The exerted force $F=ma=600\times \left(10/3\right)=2000N$

Work done $W=F\times S=2000\times 60=120000J$