Friction is a force that opposes motion. It is present whenever ... that are moving over each other, some kinetic energy is transformed into heat energy. Friction can sometimes be useful.

The heat energy from the water makes the water molecules in the hot water move faster than the water molecules in the cold water. The faster moving molecules in the hot water bump into the food ...

Forces are responsible for changing the motion of objects ... some of the most important interactions in nature. Energy, work and power Energy is a key principle in physics.