Question 7.3:
A child sits stationary at one end of a long trolley moving
uniformly with a speed V on a smooth horizontal floor. If the
child gets up and runs about on the trolley in any manner, what is
the speed of the CM of the (trolley + child) system?
Solution:
No change
The child is running arbitrarily on a trolley moving with velocity
v. However, the running of the child will produce no effect on
the velocity of the centre of mass of the trolley. This is because
the force due to the boy’s motion is purely internal. Internal
forces produce no effect on the motion of the bodies on which they
act. Since no external force is involved in the boy–trolley
system, the boy’s motion will produce no change in the velocity
of the centre of mass of the trolley.No change
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