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.

During slipping, the friction force opposes the direction of motion of the contact point - that is, the friction force acts to the left. As a result of the work done by this friction, the kinetic ...

Science presenter Jon Chase explains friction. He describes how friction opposes or prevents motion, and demonstrates the force in action with an experiment involving two interleaved books.

In this case, friction does work on the system. As we have discussed in class, the friction force on the block always opposes the motion of the block, therefore the work is always NEGATIVE. With this, ...