Question

A ball thrown up in vacuum returns after 12 sec. Its position after five seconds will be same as after:

• A. 7 sec
• B. 3 sec
• C. 4 sec
• D. 3.5 sec

Answer 1

The correct option is A 7 sec

$s=ut-\frac{1}{2}g{t}^2$

$\because t=12s,y=0m$

$\therefore u=\frac{1}{2}\times 10\times 12=60m/s$

for $t=5s,y=175m$

$\therefore 60t-\frac{1}{2}g{t}^2=175$

$60t-\frac{10}{2}{t}^2=175$

$5t^2-60t+175=0$

$\therefore t=5s$ or $t=7s$

$\because t=5s$ is not possible $\therefore t=7s$