15. You are at the gym, exercising on a step machine. You have one foot on each of the machine’s pedals and you move those pedals up and down as you step. The pedals always push upward on your feet, but they push upward harder on your feet while moving downward than they do while moving upward. When during this exercise is your foot transferring energy to the pedal that it is touching?


  • When that pedal is accelerating.
  • As that pedal moves downward.
  • As that pedal moves upward.
  • As that pedal moves either upward or down.