Question

A child is sitting on a stationary skateboard with a pile of rocks which she throws in one direction in order to make the skateboard travel in the opposite direction. If the rocks each have a mass of 0.95 kg and she can throw them with a speed of 19 m/s relative to the ground, determine the number of rocks she must throw per minute in order to maintain a constant average speed against a 3.4 N force of friction. (Note: because this is a rate, the answer may not be a whole number.)

Answer 1

Let K be number rocks minute

K*min/60sec*velocity*mass=force

3.4N=K/60sec/min *19m/s*.95kg
but N=kg*m/s^2

K=3.4kg*m/s^2 *60s/min*1/(19m/s*.95kg)
K=3.4*60/(19*.95) rocks/min

=about 11 rocks/min check that.