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Different Kinds of Friction

Dry friction is just like it sounds – if you’ve ever tried to shove a heavy box across the pavement, you know that it’s harder to get it started than keep it going. That’s because when you first start to shove the box, you’ve got to overcome the (stronger) static friction, but once you’re moving you are dealing with only with the (weaker) kinetic friction. Sometimes kinetic friction is called ‘sliding’ or ‘dynamic’ friction.

Engineers and scientists reduce friction by adding lubrication, like oil. Your car engine uses oil to slick up the metal-to-metal surfaces and keep the pistons moving smoothly.

One of the troubles with liquids is that they tend to heat up quickly (there’s still friction between the liquid and the solid surface!), so you have to devise a system to cool the liquid. You can also use gas as a lubrication. Hovercraft ride on a cushion of air, using air as the ‘lubrication’ between the skirt and the ground.

Fluid friction is when you have two fluids flowing up against each other, like a stream of water and a stream of oil together. Skin friction happens when an airplane flies through the air – the air rushing by the airplane body heats up the outside of the aircraft.

Contrary to earlier explanations found in textbooks, we now know that kinetic friction happens not because of surface roughness, but rather because of the chemical bonding that happens between the two surfaces.

Click here to get started with the experiments!

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