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A liquid is kept in a cylindrical vessel, which is rotated along its axis. The liquid rises at the side. If the radius of the vessel is 5cm and the frequency of rotation is 4rev/s, then the difference in the height of the liquid at the centre of the vessel and its sides is:
A) 8cm
B) 2cm
C) 40cm
D) 4cm

Answer
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Hint: This problem can be solved by realizing the fact that the change in the potential energy of the water will be equal to the change in the rotational kinetic energy of the water. The water in the centre of the cylinder loses potential energy which is gained by the water in the sides to attain the rotational kinetic energy.

Formula used:
KE=12Iω2
PE=mgh
I=mr2

Complete step by step answer:
In this problem, we must realize that the water at the centre drops in height and relatively the water at the sides rise. Since, the water in the cylinder rotates it has some rotational kinetic energy. This rotational kinetic energy is attained due to the loss of the potential energy of the water due to the change in height.
The rotational kinetic energy KE of a body of moment of inertia I about the axis of rotation is given by
KE=12Iω2 --(1)
where ω is the angular frequency of rotation.
The change in potential energy PE of a body of mass m due to a change h in the height of its position is given by
PE=mgh --(2)
Where g=9.8m/s2 is the acceleration due to gravity.
The moment of inertia I of a particle of mass m about an axis is given by
I=mr2 --(3)
Where r is the perpendicular distance of the particle from the axis.
Now, let us analyze the question.
We will consider a particle of mass m of the water at the side of the cylinder.
The axis in this case is the axis of the cylinder through its centre.
Therefore, the perpendicular distance of the particle from the axis, will be nothing but the radius r=5cm=0.05m of the cylinder. (1cm=0.01m)
The angular frequency of rotation is ω=4rev/s=4×2πrad/s=8πrad/s (1rev/s=2πrad/s)
Let the change in the height of the position of the particle at the side be h.
Let the moment of inertia of this particle about the axis be I
Let the change in the rotational kinetic energy of this particle be KE while the change in the potential energy be PE.
Using (3), we get
I=mr2 --(4)
Using (1), we get that,
KE=12Iω2 --(5)
Putting (4) in (5), we get,
KE=12(mr2)ω2 --(6)
Using (2), we get,
PE=mgh --(7)
As explained above, the magnitude of the change in the rotational kinetic energy will be equal to the magnitude of the change in the potential energy.
KE=PE --(8)
Using (6) and (7) in (8), we get,
12(mr2)ω2=mgh
12r2ω2=gh
h=ω2r22g
Putting the values of the variables in the above equation, we get,
h=(8π)2(0.05)22×9.8=64π2(25×104)2×9.8=0.08m=8cm (1m=100cm)
Hence, the required rise in height is 8cm.

So, the correct answer is “Option A”.

Note:
In this problem, a very common mistake that could have been made by the students is not changing the units for the angular frequency. The angular frequency is given in revolutions per second, whereas its SI unit is radians per second. If the proper conversion is not done, then the angular frequency will not be the correct value that can be used in the calculation and the ultimate answer will be incorrect.
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