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 0.70 kg and she can throw them with a speed of 17 m/s relative to the ground, decide the number of rocks she must throw per minute in order to maintain a constant average speed against a 3.5 N force of friction. (Note: because this is a rate, the answer might not be a whole number.) rocks/min