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Question

A stone of mass 1 kg tied to a light string of length l=103m is whirled in a circular path in a vertical plane. If the ratio of the maximum to minimum tension in the string is 4, find the speed of the stone at the highest point.

A
20 m/s
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B
10 m/s
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C
5 m/s
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D
1 m/s
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Solution

The correct option is B 10 m/s
Let the velocity at the highest point be VP
Let the velocity at the lowest point be VR

From conservation of mechanical energy,12mVp2+mg(2l)=12mVR2

Vp2+4gl=VR2

At the lowest point, T is maximum,

Using Newton's second law at the lowest point we have,

Tmaxmg=mVR2l

Tmax=mg+mVR2l

At the highest point, T is minimum.

Using Newton's second law at the highest point we have,

Tmin+mg=mVP2l

Tmin=mVP2lmg

TmaxTmin=4mVR2l+mgmV2plmg=4VR2+glVP2gl=4

We know that

VR2=VP2+4glVP2+5glVP2gl=43VP2=9gl

VP=3gl=3×10×103=10 m/s

Why this Question ?
Key point- Always remember that in a verticle circular motion, the tension is maximum at the lowest point and minimum at the highest point.



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