Which best demonstrates friction
a)an iceskater sliding across the ice without noticeably slowing down
b)a car rolling to a stop at a traffic light
c)a baseball moving in a projectile motion
d)the free fall of astronauts in orbit

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.