Q.
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?
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AIPMTAIPMT 2014Laws of Motion
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Solution:
Let F be the upthrust of the air.
As the balloon is descending down with an acceleration a, ∴mg=F=ma ... (i)
Let mass m0 be removed from the balloon so that it starts moving up with an acceleration a.
Then, F−(m−m0)g=(m−m0)a F−mg+m0g=ma−m0a ... (ii)
Adding eqn. (i) and eqn. (ii), we get m0g=2ma−m0a m0g+m0a=2ma m0(g+a)=2ma m0=a+g2ma