Question

Two frames, one stationary and the other moving, are initially coincident. Two observers in the two frames observe a body initially at rest in the coincident frame. A constant force F starts acting on the body along horizontal axis when moving frame starts separation from fixed frame. Work done 'W' as observed by stationary frame and W as observed from moving frame are compared to each other as

• A. W= W'
• B. W = W'
• C. W = $\frac { 1 } { 2 }$W'
• D. W = 2W'

Answer 1

Answer: (B)

W = W'

Answer 2

Let u be the speed of moving frame and horizontal distance covered at any instant from moving frame is x then distance from stationary frame is (x + ut)

Work done w.r.t stationary frame W

= F(x + ut)= Fx + Fut

But Fx is the work done w.r.t. moving frame

∴ W = W' + Fut

i.e., W ≠ W'