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

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Solution:

Let be the upthrust of the air.
As the balloon is descending down with an acceleration a,
... (i)
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Let mass be removed from the balloon so that it starts moving up with an acceleration .
Then,
... (ii)
Adding eqn. (i) and eqn. (ii), we get