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.)

You have two photos of a person walking. one shows the person at the corner of third and main streets, the other shows the person at the corner of tenth and main streets. there are lampposts at every corner in this town, and the first picture shows it to be 10: 32: 00 exactly. the second picture shows it to be 10: 49: 30. you know three facts: (1) all of the clocks are synchronized; (2) there are exactly 12 equal-sized blocks per kilometer in this town; and (3) the streets that cross main in this area are numbered consecutively, with no interruptions. what is the person’s average speed in kilometers per hour?