Question

An object of mass m is tied to a string of length l and a variable force F is applied on it which brings the string gradually at angle theta with the vertical. Find the workdone by the force F.

Answer 1

Explanation:

three forces are acting on the object

1. tension (T)

2. weight (mg)

3. applied force (F)

using work energy theorem W net= ∆KE or W r +W mg +W F =0.... (1)

∆KE=0

because K i =K f= 0

W T=0 as tension is always perpendicular to displacement

W mg=-mgh or W mg =-mgl(1-cos theta)

therefore W F = mgl(1- cos theta)