Friction is a force that opposes motion. It is present whenever two surfaces rub over each other, such as when you rub your hands together, or when you apply the brakes on a bike or in a car.

With their whip-like tails, human sperm propel themselves through viscous fluids, seemingly in defiance of Newton's third law ...

Atoms can exist on their own or as molecules. Forces are responsible for changing the motion of objects. If more than one force is present, the shape of an object can also be changed. Momentum ...