First slide

Second law

Question

A ping-pong ball of mass m is floating in air by a jet of water emerging out of a nozzle. If the water strikes the ping-pong ball with a speed v and just after collision water falls dead, the rate of mass flow of water in the nozzle is equal to:

Moderate

A

2mg / v
B

mv /g
C

mg / v
D

None of these

Solution

The impact force F = Δp / Δt where Δp = change of momentum of water of mass Δm striking the ball a speed v during time Δt . Since water falls dead after collision with the ping-pong ball
Δp = Δmv ⇒ F = v (Δm /Δt) upward on the ball
Where Δm/Δt = rate of mass of water in the nozzle.
Since the ball is in equilibrium.
F - mg = 0 ⇒ F = mg
⇒ V . (Δm/Δt) = mg ⇒ Δm/Δt=mg/V
Hence (3) is the corerct choice

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