Question

A car starts from rest and accelerates at uniform rate of $5m/s^2$ for some time then moves with contant speed for some time and retards at the same uniform rate and comes to rest. Total time for the journey is 25 sec and average speed for the journey is 20 m/S. How long does the car move with constant speed

• A. 10 sec
• B. 12 sec
• C. 15 sec
• D. 18 sec

Answer 1

Answer: (C)

$S=S_1+S_2+S_3$
$20\times 25=\frac{1}{2}\times 5\times {\left(\frac{25-t}{2}\right)}^2\times 2+5\left(\frac{25-t}{2}\right)t$
$\therefore$ $t$ = 15 s