Question

10. A child is
(3) 3
A child is standing at one end of a long trolley moving with a speed v on a smooth horizontal track. If the
child starts running towards the other end of the trolley with a speed u, the centre of mass of the system
(trolley + child) will move with a speed
(1) Zero
(3) v
(2) (v + u)
(4) (v – u)​

Answer 1

Answer:zero

Explanation:

Center of mass will remain unchanged until the angle of child is changed

Answer 2

Answer:

The centre of mass of the system (trolley + child) will move with a speed of v.

Explanation:

We considered (trolley + child) as a single system. Now the trolley is moving with a speed v which is a constant. That means, the momentum is constant. Which in turn means that the external force acting on the system is zero.

Now, let us consider the system's initial velocity to be moving with v. Now, in order to conserve the momentum and let the centre of mass at the same position as it were to be, the final velocity has to be the same as that of initial but with an opposite direction.

Hence, the system will move with a speed v.

Hope it helps! ;-))