On a train moving along east with a constant speed v, a boy revolves a bob with string of length l on smooth surface of a train, with equal constant speed v relative to train. Mark the correct option(s).

Options

A. Maximum speed of bob is 2v in ground frame.
B. Tension is string conneting bob is `(4mv^(2))/l` at an instant.
C. Tension in strings is `mv^(2)/l` at all the moments
D. Minimum speed of bob is zero in ground frame.

Answer

A, C, D

Explanation

`vec(v)_(b//t)+vec(v)_(T//g)=vec(v)_(b//g)` for `v_(b//g)` maximum
`vec(v)_(bT)` and `vec(v)_(T//g)` most be parallel `vec(v)_(b//g)=vec(v)+vec(v)=2vec(v)`
`v_(b//g)=2v`
bob performing circular motion in frame of train
`T=(mv^(2))/l`
for `vec(v)_(b//g)` is minimum
`vec(v)_(b//T)` and `vec(T)_(T//g)` must be antiparallel
`vec(v)_(b//g)=vec(0)`