How Many Rocks Must a Child Throw to Maintain Constant Speed in a Wagon?

[bkoz316]

Remain at constant speed?

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?



Not sure on what to do...please help!


Thanks!

 

[PhanthomJay]

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?



Not sure on what to do...please help!


Thanks!

You have to determine what force the child must exert the wagon in order to make it move at constant speed. Then that force can be equated to the momentum change caused by the changing mass (Hint: F= rate of change of momentum, per Newton's 2nd law).