Friction and air resistance

What friction is

Friction is a contact force that acts when two surfaces rub or try to slide past each other. It always acts against the motion (or against the direction something is trying to move), so it is a slowing-down force.

Friction happens because no surface is perfectly smooth. Under a microscope, even polished metal is covered in tiny bumps and hollows. When two surfaces press together, the bumps catch and stick to each other, and they have to be dragged over or broken off for the surfaces to slide.

There are two kinds to know. Static friction holds a still object in place, like a book on a sloping desk. It can only grip up to a certain limit. Once you push harder than that, the object starts to slide, and sliding friction takes over, which is a bit weaker. That's why it takes a big shove to get a heavy box moving, but less to keep it going.

Friction makes heat

Rub your hands together fast and they warm up. Whenever friction slows something down, the energy of the motion is turned into heat (and often sound). That is useful when you light a match or warm your hands. It's a problem in engines, where moving parts would overheat and wear out without oil, and in brakes, which can glow red-hot on a racing car. Meteors streak across the sky because they slam into the air so fast that the air in front of them is squashed and heated, and the rock glows and burns up.

What affects friction?

• The surfaces: rough surfaces like sandpaper or rubber grip far more than smooth ice or oiled metal.
• How hard the surfaces press together: a heavier box, or a box with another box on top, has more friction. A heavier object has more grip but also more weight to move, which is why a heavy and a light block of the same material start to slide down a ramp at the same angle.
• What's in between: water, oil, wax, sand or dust can change friction a lot. Wet roads are slippery; sand on an icy footpath stops people falling.

Useful friction and unwanted friction

To increase friction: use rough or grippy materials, add tread to tyres, sprinkle sand or grit, put chains on tyres in snow.

To reduce friction:

• Lubricants such as oil, grease, wax or soap coat the surfaces so the bumps slide over each other. Surfers wax their boards for grip, but skiers wax their skis to slide faster: different waxes, opposite jobs.
• Rollers, wheels and ball bearings roll instead of sliding. The ancient Egyptians may have moved giant stone blocks on rollers; today, tiny steel balls in bearings let bike wheels and skateboard wheels spin freely.
• A cushion of air or magnetism keeps surfaces apart completely. A hovercraft floats on air. Maglev trains hover above the track using powerful magnets; a test maglev in Japan reached 603 km/h in 2015.

Air resistance and streamlining

Air resistance (also called drag) is friction between a moving object and the air. The faster you go and the bigger your front surface, the more air you have to push out of the way, so the bigger the drag. Water resistance works the same way, only more strongly.

• Parachutes have a huge surface to make as much drag as possible.
• Cars, planes, racing bikes and swimsuits are streamlined: smooth and rounded so air or water flows around them easily. Dolphins, penguins and sharks are naturally streamlined; a slow-moving seahorse isn't.
• Cyclists in a race ride close behind each other in a pack. The rider behind is shielded from the wind and can use about 30% less energy, which is why teams take turns at the front.