Question

A rocket travelling at a speed of $200 \,m / s$ ejects its products of combustion at the speed of $1200\, m / s$ relative to the rocket, then the speed of escaping vapours with respect to the person on the ground is:

• A. $1000 \,ms ^{-1}$
• B. $1200\, m / s$
• C. $1400\, m / s$
• D. $200\, m / s$

Answer 1

Answer: (A)

Relative velocity of combustion product of rocket
w.r.t the motion of rocket
$\vec{V}_{c}=+1200 m / s \quad \overrightarrow{ V }_{ r }=-200 m / s$
Velocity of vapours is $V _{ v }$
$\overrightarrow{ V }_{ c }=\overrightarrow{ V }_{ v }-\left(\overrightarrow{ V }_{ r }\right)$
$1200=\bar{V}_{v}-(-200)$
$\overrightarrow{ V _{ v }}=1000 m / s$