Achild 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 0.40 kg and she can throw them with a speed of 14 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 2.1 n force of friction. (note: because this is a rate, the answer may not be a whole number.)