Conservation of Energy in different frame

[Microzero]

A train moves in a straight at a constant speed u. A boy on the train throw a ball of mass m straight ahead, along the motion of the train, with a speed v with respect to himdelf.
a)What is the KE gain of the ball measured by the boy?
b)What is the KE gain of the ball measured by a man standing by the railroad track?
c)How much work is done by the boy throwing the ball?
d)How much work is done by the train?



a) 1/2 mv^2

b) 1/2m(v+u)^2 - 1/2 mu^2 = 1/2mv^2 +muv

c) measured by the boy: 1/2 mv^2 , measured by the man: 1/2 mv^2

d) measured by the boy: 0 , measured by the man: muv

My fd suggest that there is no workdone by the boy measured by the man, all work is done by the train.

Is there anything wrong in part c and d?
Plz tell me why if I am wrong

 

[member 553083]

.No, your answers for part c and d are correct. The boy does the same amount of work measured by himself and measured by the man standing by the railroad track, which is 1/2 mv^2. The work done by the train is the difference between the kinetic energy gain of the ball measured by the man on the railroad track and the kinetic energy gain of the ball measured by the boy, which is muv.