Motion in a Straight Line - Result Question 39
40. A man of $50 kg$ mass is standing in a gravity free space at a height of $10 m$ above the floor. $He$ throws a stone of $0.5 kg$ mass downwards with a speed $2 m / s$. When the stone reaches the floor, the distance of the man above the floor will be:
(a) $9.9 m$
(b) $10.1 m$
(c) $10 m$
(d) $20 m$
[2010]
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Answer:
Correct Answer: 40. (b)
Solution:
- (b) No external force is acting, therefore, momentum is conserved.
By momentum conservation,
$50 u+0.5 \times 2=0$
where $u$ is the velocity of man.
$u=-\frac{1}{50} ms^{-1}$
Negative sign of $u$ shows that man moves upward.
Time taken by the stone to reach the ground
$=\frac{10}{2}=5 sec$
tance moved by the man $=5 \times \frac{1}{50}=0.1 m$
$\therefore \quad$ when the stone reaches the floor, the distance of the man above floor $=10.1 m$
