Circular MotionHard

Question

A stone of mass 1 kg tied to a light inextensible string of length L = 10/3 m, whirling in a circular path in a vertical plane. The ratio of maximum tension in the string to the minimum tension in the string is 4, If g is taken to be 10 m/s2, the speed of the stone at the highest point of the circle is :

Options

A.10 m/s
B.5√2 m/s
C.10√3 m/s
D.20 m/s

Solution

Maximum tension in string at lowest  
Tmax = − mg      .....(1)
maximum tension in string at heighest point.
 Tmin = − mg      .....(2)
from energy conservation
mv2LP = 2mgL + mv2HP      .....(3)
from (1) & (3)
Tmax = + 5 mg         ...(4)
from (2) & (4)
4 = = ⇒   3mv3HP 9 mgL
⇒    VHP = = 10 m/s    Ans.

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