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Grade: 11


How to resolve it`s FBD & plz tell if there any short trick to handle such Qns during JEE.

4 years ago

Answers : (1)

Manas Shukla
102 Points
Why do u even need an FBD for such type of problems?
As given 
\sigma =5g/cc = 5000kg/m^{3}
\rho = 1000kg/m^{3}
Clearly the density of cylinder is more than the density if water so maximum speed will be when the cylinder is just fully submerged in water. So center of mass has to travel 5m for that to happen.
Now when the cylinder is submerged in water there is minimal rise in potential energy of water because lake is deep and very big compared to cylinder.
Total loss in potential energy = gain in kinetic energy.
(\rho + \sigma ) 5g = \frac{1}{2} \sigma v^{2}
since the volume from each side gets cancelled.
Solving we get , 
v = \sqrt{120}
Remember if the problem looks too difficult to solve using balancing forces , try solving using energy.
Approve if it helps thanks.
4 years ago
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