Average Speed.......I Need Help.........?

Question

A child sits in a wagon with a pile of 0.47 kg rocks. If she can throw each rock with a speed of 9 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 3.5 N force of friction?







Thanks so much for your time!!!

Answer 1

I'm afraid I don't understand "a constant average speed". Each throw is going to give the wagon a speed based on conservation of momentum.
Momentum before = momentum after.
0 = 0.47 kg*9 m/s - momentum of wagon
So rearranging
momentum of wagon = 0.47 kg*9 m/s = 4.23 kg*m/s

The wagon's velocity is going to decrease until she throws another rock. So the speed of the wagon will increase/decrease/increase/decrease. Maybe she should throw another just before the friction brings her to a stop. OK, assume that's the plan.

Momentum can be said to be equal to the impulse, F*t, necessary to bring the object to a stop. So if
F*t = 4.23 kg*m/s
and you know F = 3.5 N, then
t = 4.23 kg*m/s / 3.5 N

OK, for you to do anything with that, you need to know that those 3.5 N can be written 3.5 kg*m/s^2.

You can solve for t from there. Every t seconds she needs to throw another.