Man Jumping from Trolley
Question:
For that man and trolley example, it seems that the man pushing
on the trolley will have no effect on the whole system with man
and trolley as a one part. However, if the floor of the trolley
where the man stands on is frictionless, then there will be no
'balancing force' acts on the man. If we consider the
man-trolley system again, then it seems that the trolley will
move, unless the man only apply a force with one of his hand and
a reaction is gained by the other hand. However, if we just
consider the trolley but exclude the man as a system, will the
trolley move due to no reaction from friction between the floor
and the man?
Just reply when you have time! Thanks very very very
much!!!
Answer:
You are correct that in the man-trolley system, if the man pushes
off from the trolley with no friction between them, the man will
fly off in the direction in which he pushed and the trolley will
move off in the other direction with initial velocity
proportional to the ratio of the masses. Since the man in this
example transfers energy to the trolley, we must take as a system
the man-trolley combination. Otherwise we would observe the
trolley to move spontaneously, violating the conservation of
momentum. If you ever make a measurement which indicates that the
conservation of momentum is violated, you need to look again at
the way the system is defined.
It was exactly in this way that the sub-atomic particle the
neutrino was discovered. The conservation of momentum in
radioactive beta decay appeared to be violated. To prevent this
it was necessary to postulate an undiscovered particle.