Question

A rocket is fired upward from the earth's surface such that it creates an acceleration of 19.6 m/sec². If after 5 sec its engine is switched off, the maximum height of the rocket from earth's surface would be

• A. 245 m
• B. 490 m
• C. 980 m
• D. 735 m

Answer 1

Answer: (D)

735 m

Answer 2

Given $a = 19.6\mspace{6mu} m/s^{2} = 2g$

Resultant velocity of the rocket after 5 sec

$v = 2g \times 5 = 10g\mspace{6mu} m/s$

Height achieved after 5 sec, $h_{1} = \frac{1}{2} \times 2g \times 25 = 245m$

On switching off the engine it goes up to height $h_{2}$ where its velocity becomes zero.

$0 = (10g)^{2} - 2gh_{2} \Rightarrow h_{2} = 490m$

$\therefore$Total height of rocket $= 245 + 490 = 735\mspace{6mu} m$