CogKnoSys

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Physical

Friction

Friction is the force that resists motion when surfaces, fluid layers or parts of a material slide or grind against each other.

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The idea

Friction is the force that resists motion when solid surfaces, layers of fluid or parts inside a material slide or grind against each other. It is why a ball rolling across grass slows down and stops. It is also why you can walk without slipping and why tires can grip the road.

There are several types, including dry, fluid, lubricated, skin and internal friction. Dry friction acts between two solids, such as a box sliding across a floor. Fluid friction slows things moving through water or air, and a lubricant such as oil adds a slippery layer that lowers friction between parts. The study of friction and wear is called tribology, and it has a history of more than 2,000 years.

When surfaces rub, some of the energy of motion turns into heat. That is why rubbing your hands warms them, and why rubbing pieces of wood together can start a fire. Friction also causes wear, which can damage machine parts over time. Losses to friction account for about 20% of all the energy used in the world. Because so many small effects add up, scientists usually study friction through experiments rather than working it out from basic principles.

Check yourself

Question 1 of 2

What is the study of friction and wear called?

Pick one. The first pick counts.

Composition

what it's made of

  • Surface Bumps — Tiny bumps on surfaces catch and bend as they slide past each other.
  • Pressing Force — The harder two surfaces are pushed together, the stronger the friction between them.
  • Lubricants — Layers of oil, grease or water that separate surfaces and lower friction.
  • Heat — Energy of motion that friction turns into warmth.
  • Wear — Gradual damage to surfaces that rub against each other.

Connection

how it relates

Under Motion

Cause and effect

  1. Thermal Energystrength 70 of 100

    Friction turns some of the energy of motion into heat, which is thermal energy.

Related

  1. Materialsstrength 60 of 100

    How rough or smooth a material is affects how much friction it creates.

  2. Joint Typesstrength 50 of 100

    Movable joints use smooth cartilage and slippery fluid to keep friction low as bones move.

Change

how and why it evolves

Past
In ancient times, people made fire by rubbing pieces of wood together, using friction to create heat.
Now
Today tribologists study friction and wear to design better engines, tires, bearings and lubricants.
Next
Because friction wastes so much energy, research into slicker surfaces and better lubricants will likely keep growing.

Knowledge you can walk. Read the idea, then step up, across or down.

Friction

Friction is the force that resists motion when surfaces, fluid layers or parts of a material slide or grind against each other.

Learn, discuss, create, buy or invest

The idea

Friction is the force that resists motion when solid surfaces, layers of fluid or parts inside a material slide or grind against each other. It is why a ball rolling across grass slows down and stops. It is also why you can walk without slipping and why tires can grip the road.

There are several types, including dry, fluid, lubricated, skin and internal friction. Dry friction acts between two solids, such as a box sliding across a floor. Fluid friction slows things moving through water or air, and a lubricant such as oil adds a slippery layer that lowers friction between parts. The study of friction and wear is called tribology, and it has a history of more than 2,000 years.

When surfaces rub, some of the energy of motion turns into heat. That is why rubbing your hands warms them, and why rubbing pieces of wood together can start a fire. Friction also causes wear, which can damage machine parts over time. Losses to friction account for about 20% of all the energy used in the world. Because so many small effects add up, scientists usually study friction through experiments rather than working it out from basic principles.

Check yourself

2 quick questions, instant feedback.