Q. An astronaut floating freely in space decides to use his flash light as a rocket. He shines a $10\ watt$ light beam in a fixed direction so that he acquires momentum in the opposite direction. If his mass is $80\ kg$, how long must he need to reach a velocity of $1\ ms^{- 1}$
A) $9\ s$
B) $2.4 \times 10^{3}\ s$
C) $2.4 \times 10^{6}\ s$
D) $2.4 \times 10^{9}\ s$
Explanation
(d) Let it take $t\ sec$ for astronaut to acquire a velocity of $1\ ms^{- 1}$. Then energy of photons $= 10\ t$
Momentum $= \frac{10t}{C} = 80 \times 1$
$t = \frac{80 \times 1 \times 3 \times 10^{8}}{10} = 2.4 \times 10^{9}\sec$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.