It requires 20 turns of the stem of a watch to wind the main spring and this stores sufficient energy to keep the watch running for `30 hrts.` If 10 turns are given to thestem, the watch will run for

Options

A. `20 h`
B. `24 h `
C. `7.5 h`
D. `10 h`

Answer

C

Explanation

The watch runs on account of potential energy stored by winding the spring .
As `P.E. prop x^(2)`
and `x` is halved (from 20 turns to 10 turns )
`:. P.E.` becomes `(1)/(4)th.`
The watch will run for `(30)/(4)=7.5` hours