Friction is the resistance to motion that occurs when one material slides across another material. all materials have surface irregularities and are not perfectly smooth.

as the materials move, the irregularities get caught on one another requiring more force. friction wears down a surface while producing heat, such as tires on a road or skin receiving a rug burn from sliding across carpet.

1. why would someone rub their hands together really fast when they are cold?

2. give another example of friction that was not discussed above.