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Question

A stone is fastened to one end of a string and is whirled in a vertical circle of radius 10 m. Find the minimum speed the stone can have at the highest point on the circle.
[Takeg=10 m/s2]

A
10 m/s
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B
12 m/s
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C
5 m/s
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D
0
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Solution

The correct option is A 10 m/s
At the highest point, forces acting on the stone are shown below.



So, at the highest point, net force towards the centre is given by

Fcentre=T+mg

Let the minimum speed at highest point is v.

mv2R=T+mg

For v to be minimum, T should be minimum, i.e. zero

T=0

mg=mv2R

v=Rg=10×10

v=10 m/s

Therefore, option (A) is correct.
Why this question ?
Tip: In vertical circle motion, at the highest point, required centripetal force is provided by gravitational force only.

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