A child sits in a wagon with a pile of 0.60 kg rocks. If she can throw each rock with a speed of 10 m/s relative to the ground, causing the wagon to move, how many rocks must she throw per minute to maintain a constant average speed against a 3.7 N force of friction?

Question

A child sits in a wagon with a pile of 0.60 kg rocks. If she can throw each rock with a speed of 10  m/s relative to the ground, causing the wagon to move, how many rocks must she throw per minute to maintain a constant average speed against a 3.7 N force of friction?

I got .616 rocks/min but that isnt right.

drwls · Answer

To produce a stady reaction force of 3.7 Newtons, momentum must be thrown out in the opposite direction at a rate dm/dt given by
(dm/dt)*10 m/s = 3.7 N
dm/dt = 0.37 kg/s

That is 0.616 rocks per second, NOT per minute