Apush or pull that gives energy to an ob-
ject, causing that object to start moving, stop
moving, or change its motion is
1. friction.
2. pressure.
3. inertia.
4. a force.

Air is contained in a variable-load piston-cylinder device equipped with a paddle wheel. initially, air is at 400 kpa and 17°c. the paddle wheel is now turned by an external electric motor until 75 kj/kg of work has been transferred to air. during this process, heat is transferred to maintain a constant air temperature while allowing the gas volume to triple. calculate the required amount of heat transfer in kj/kg.