A 500 kg rocket is set of vertical firing. The relative speed of burnt gas is `800ms^(-1)` To give an initial upwards acceleration of `20ms^(-2)`, the amount of gas ejected per second to supply the needed thrust will be

Options

A. `127.5 kg s^(-1)`
B. `187.5 kg s^(-1)`
C. `185.5 kg s^(-1)`
D. `137.5 kg s^(-1)`

Explanation

`F_("thrust")-mg=ma`
`m=5000kg`
`a=20m//s^(2)`
`rArrF_("thrust")=U_(rel)=15000N`
`F_("thrust")=U_(rel)xx(dm)/(dt)`
`rArr(-800)xx(dm)/(dt)=15000`
`rArr(dm)/(dt)=-187.5 kg//s`