A particle is moving in a circular path in the vertical plane. It is attached at one end of a string of length `l` whose other end is fixed. The velocity at lowest point is `u`. The tension in the string `T` and acceleration of the particle is a any position. Then `T` is zero at highest point if

Options

A. `ugtsqrt(5gl)`
B. `u=sqrt(5gl)`
C. Both (a) and (b) are correct
D. Both (a) and (b) are wrong

Answer

B

Explanation

If `ugtsqrt(5gl)` , then `T` and `a` are in same direction.
hance, `T` . `a` is positive.
If `u=sqrt(5gl)` , then `T=0`
:. `T.a=0`