Describing friction

When two surfaces are in contact there is a force acting on each surface that acts in a direction to stop them moving past one another. This is often said to be due to friction, but there are two possibilities: movement, or no movement.

Rough surfaces have more friction than smooth surfaces and liquids such as oil or water are sometimes used as lubricants to reduce the effect of friction. There is no mystery behind friction. It acts on objects at the surfaces as to prevent or reduce movement between the surfaces. When friction prevent sliding there is grip, when sliding is reduced there is slip. There is enough of a difference between these two that we suggest that you distinguish between grip forces and slip forces.

Grip forces are often a good thing. Without grip we couldn't walk anywhere. When we walk the grip force between our shoes and the ground enables us to push against the ground. On an icy surface walking is much harder because there is less almost no grip force.

Slip forces are useful in reducing existing motion. Without them the options for stopping a bicycle would be limited.

For a simple explanation of how grip or slip forces happen, imagine two surfaces at a microscopic level. All surfaces are full of imperfections. Nothing is totally smooth. When these imperfections catch on each other they act to prevent or reduce movement. Just think about rubbing two sandpaper surfaces together.

Friction through forces spectacles

Through forces spectacles a grip force or slip force is shown by drawing in a force arrow, parallel to the surface resting on the rough solid surroundings, to remind you of the origins of the force on the object. In the example below, the force acting on the box is shown by an arrow along the bottom surface of the box in the opposite direction to the intended motion.