Question

A rocket is fired vertically from the ground. It moves upwards with a constant acceleration 10 $m{s}^{-2}$ for 30 seconds, after which the fuel is consumed. After what time from the instant of firing, the rocket will attain the maximum height? (Take g = 10 $m{s}^{-2}$)

• A. 75 s
• B. 60 s
• C. 45 s
• D. 30 s

Answer 1

The correct option is B 60 s

$\begin{array}{c}S=ut=\frac{1}{2}a{t}^2\\ S=\frac{1}{2}\left(10\right){\left(30\right)}^2=4500m\\ v^2-u^2=2as\\ v=300m/s\end{array}$

now after fuel is consumed

$\begin{array}{c}a=-10\\ v=0\\ v=u-gt\\ t=30s\\ totaltime=30+30=60s\end{array}$