Friction

When you rub your hands together, they warm up. That warming is caused by friction. Friction is a force that opposes motion.

What is friction?

Solid friction is the force between two touching surfaces that resists sliding. It can slow things down and produce heating.

Drag is friction on an object moving through a fluid (a liquid or a gas). In air it is called air resistance; in water it is water resistance.

Friction in solids

• Acts along the surfaces, opposite to the direction of motion (or attempted motion).
• Bigger when surfaces are rougher and when they are pressed together more firmly.
• Produces heat, so surfaces can warm up and wear down.

Friction in liquids and gases (drag)

• Opposes motion through the fluid.
• Gets larger at higher speeds.
• Depends on shape and area facing the flow (streamlined shapes reduce drag).

Effects and uses

• Useful: grip when walking, car tyres on roads, brakes stopping wheels.
• Unwanted: energy wasted as heat in machines, parts wearing out.

Reducing or increasing friction

• Reduce: lubrication (oil/grease), smoothing surfaces, using wheels or ball bearings, streamlining shapes.
• Increase: rougher surfaces, treads on shoes/tyres, pressing surfaces together more (greater normal force).

Work and energy with friction

When you push something at steady speed, your push balances friction. The work you do against friction becomes thermal energy. The equation is $W = F d$, and $W = \Delta E$ (work done equals energy transferred).

Common misconceptions

• Friction can act even before something moves (it resists starting to slide).
• Air resistance and water resistance always act opposite to the motion, not always “upwards”.