If two objects have the same mass, we can know they are made of the same substance. If two objects have the same volume, we can know they are made of the same substance. If two objects have the same density, we can know they are made of the same substance.

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.