Question
A $$5000\,kg$$ rocket is set for vertical firing. The exhaust speed is $$800\,m{s^{ - 1}}.$$ To give an initial upward acceleration of $$20\,m{s^{ - 2}},$$ the amount of gas ejected per second to supply the needed thrust will be $$\left( {g = 10\,m{s^{ - 2}}} \right)$$
A.
$$127.5\,kg\,{s^{ - 1}}$$
B.
$$187.5\,kg\,{s^{ - 1}}$$
C.
$$185.5\,kg\,{s^{ - 1}}$$
D.
$$137.5\,kg\,{s^{ - 1}}$$
Answer :
$$187.5\,kg\,{s^{ - 1}}$$
Solution :
Thrust force on the rocket $${F_t} = {v_r}\left( { - \frac{{dm}}{{dt}}} \right)\,\,\left( {{\text{upwards}}} \right)$$
Weight of the rocket $$w = mg\,\,\left( {{\text{downwards}}} \right)$$
Net force on the rocket $${F_{{\text{net}}}} = {F_t} - w$$
$$\eqalign{
& \Rightarrow ma = {v_r}\left( {\frac{{ - dm}}{{dt}}} \right) - mg \cr
& \Rightarrow \left( {\frac{{ - dm}}{{dt}}} \right) = \frac{{m\left( {g + a} \right)}}{{{v_r}}} \cr} $$
∴ Rate of gas ejected per second $$ = \frac{{5000\left( {10 + 20} \right)}}{{800}}$$
$$\eqalign{
& = \frac{{5000 \times 30}}{{800}} \cr
& = 187.5\,kg\,{s^{ - 1}} \cr} $$