Question

A rocket is launched to travel vertically upward with a constant velocity of 20 m/s. After travelling 35 s the rocket develops a snag and its fuel supply is cut off. The rocket then travels like a free body, the height achieved it is

• A. 600 m
• B. 720 m
• C. 800 m
• D. 820 m

Answer 1

The correct option is B 720 m

For upward motion of rocket with uniform velocity $u=20m{s}^{-1};t=35s;$
$h=ut=20\times 35=700m$
For upward motion after fuel cut off $u=20m{s}^{-1};v=0;g=-10m{s}^{-2}$
using formula $v=u+gt$
$0=20-10\times t$
$t=2s$
from formula $v^2=u^2+2gh$
putting values,we get,
$0={20}^2-2\times 10\times h$
$0=400-20h$
$h=\frac{400}{20}=20m$
Total height achieved $=700+20=720m$
hence,option b is correct.