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

Options

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

Answer

A

Explanation

Speed of rocket after 5 s.
v=u-gt
0=u-9.8`xx5`
=49m/s
from `h=ut-(1)/(2)g t^(2)`
`=0-(1)/(2)xx9.8(5)^(2)`
`=(245)/(2)m`
when engine is turned off
`v^(2)=u^(2)-2gh`
`0=u^(2)-2gh`
`h=(u^(2))/(2g)=(49xx49)/(2xx9.8)=(245)/(2)m`
Maximum height from earth surface
`=h_(1)+h_(2)=(245)/(2)+(245)/(2)=245m`