How many rocks/min to mantain a constant speed

[tironci]

This is the problem i am trying to solve:

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 2.0-N force of friction? I need to find change in m/ change in t for rocks/min

m/t = thrust/v , now i found thrust = (m/t)v -> (0.6kg/60s) 10m/s = 0.1 N

m/t = thrust/ v -> 0.1/10 = 0.01 kg/s if i turn this it kg/min 0.01/60 = 1.7x10^-4

but this doesn't seem to be the right correct answer?

Any help would be appreciated

 

[unscientific]

F = v(dm/dt)

2 = 10(dm/dt)
dm/dt = 0.2 (thats per second)