Question

A particle starts with a velocity $200cm/s$ and moves in a straight line with a retardation of $10cm/s^2$. Its displacement will be $1500cm$:

• A. Only once at $30s$ from start
• B. Only once at $10s$
• C. Twice at $10s$ and $30s$
• D. Always

Answer 1

Answer: (C)

Twice at $10s$ and $30s$

$v=200$ cm/s
$a=-10cm/s^2$
$s=1500$cm

$s=ut+\frac{1}{2}a{t}^2$
$1500=200t-1/2\times 10t^2$
$150=20t-t^2/2$
$t^2/2-20t+150=0$
$\Rightarrow t=10$ s & $30$ s