You push a friend who is sitting on a swing. The mass of your friend is 45 kg and accelerates at a rate of 4 m/s2. What is the force you exerted?

Which of the following statements is true? a. most heat energy is easily recovered and used for useful actions. b. friction causes molecules to vibrate more slowly. burning air and gasoline in an c. engine changes chemical energy into mechanical energy. it is impossible to d. change mechanical energy into mechanical energy.