Technically it would be mass or speed because it depends on ow big or small it is, but if you break it down it will be due to gravity as it gets stronger
Definitiely a turn of a wheel. Skiing down a ski slope is going down at an angle and it's usually straight. The turn of a wheel just makes a lot more sense
This is an example Newton's Third Law. All the kinectic energy from the moving car transferred the potential energy of the parked car. This potential is not much since the brakes are on (hopefully) and it's not in a non-moving position.