Laws of Motion - Result Question 24

24. A balloon with mass ’ $m$ ’ is descending down with an acceleration ’ $a$ ’ (where $a<g)$. How much mass should be removed from it so that it starts moving up with an acceleration ‘a’?

[2014]

(a) $\frac{2 m a}{g+a}$

(b) $\frac{2 ma}{g-a}$

(c) $\frac{ma}{g+a}$

(d) $\frac{ma}{g-a}$

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Answer:

Correct Answer: 24. (a)

Solution:

  1. (a) Let upthrust of air be $F_a$ then For downward motion of balloon,

$F_a=mg-ma \Rightarrow F a=m(g-a)$

For upward motion,

$ \begin{equation*} F_a-(m-\Delta m) g=(m-\Delta m) a \tag{ii} \end{equation*} $

From equation (i) and (ii),

Therefore, $\Delta m=\frac{2 m a}{(g+a)}$