Friction facts for kids

Friction is a force that resists motion between two surfaces that are touching each other. It's like a tiny invisible brake that slows things down. When two surfaces rub against each other, tiny bumps and irregularities on those surfaces get in each other's way. This creates resistance, and that resistance is friction.

The energy lost to friction is turned into sound and heat. Two kinds of friction are static and kinetic. Static friction is when the friction is strong enough to stop movement between two objects. Kinetic friction is used when the frictional force is not strong enough to stop all motion. An effect of kinetic friction is heat production.

Many everyday mechanisms that utilize friction include car brakes, suitcases, and shoes. Even a person sliding across the floor uses a form of friction.

Model of asperities

Some facts about friction

• Friction can be thought to be caused by bumps on two touching surfaces. These bumps are called asperities. As these two rough surfaces move against each other they get stuck and prevent motion until the asperities are broken or slid over. Even the smoothest materials have these asperities.

• As the force that is pushing on an object increases, the static friction increases too. However, if the force gets bigger than the maximum static friction, it makes the object accelerate. The object will begin to accelerate as soon as the static friction is less than the force pushing on it. After that point kinetic friction will act on the object.

• The coefficient of friction is the ratio between the force slowing something down and the force pushing them together. The larger it is the stronger friction is (and the more the object is slowed down). For example, a person's skates on ice have a small coefficient of friction, but rubber on concrete has a big one.

• Internal friction in a fluid is called viscosity. Viscosity is how hard it is for the fluid to move or flow. For example, honey is more "viscous" than water.

Reducing friction

We often want to reduce friction to make things move more easily. Here are some ways we do that:

• Lubricants: Lubricants, like oil or grease, are slippery substances that reduce friction between moving parts. They create a thin layer between the surfaces, preventing them from rubbing directly against each other. The use of lubricants dates back to ancient times, with evidence suggesting the use of animal fats and oils in ancient Egypt as early as 3000 BC.

• Streamlining: Streamlining is designing objects to reduce fluid friction. Think about the shape of a fish or a bird – their bodies are designed to move through water or air with minimal resistance. The development of streamlined designs has been crucial in the advancement of transportation, from cars and airplanes to boats and trains. The principles of streamlining were understood and applied to a limited extent in ancient times, but significant advancements came with the development of aerodynamics in the 20th century.

• Ball bearings: Ball bearings are small, hard spheres that reduce friction between rotating parts. They allow one surface to roll over another instead of sliding, significantly reducing friction. The earliest known use of ball bearings dates back to the 1st century AD, though their widespread use came much later.

Friction in everyday life

Friction is everywhere. It's involved in almost everything we do:

• Walking: Friction between your shoes and the ground allows you to walk without slipping.

• Writing: Friction between the pen and the paper allows you to write.

• Driving: Friction between the tires and the road allows you to drive.

• Braking: Friction between the brake pads and the wheels allows you to stop.

• Playing sports: Friction between your hands and a ball allows you to grip it.

Related pages

• Laminar flow
• Force
• Movement

Images for kids

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  Figure 1: Simulated blocks with fractal rough surfaces, exhibiting static frictional interactions

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  When the mass is not moving, the object experiences static friction. The friction increases as the applied force increases until the block moves. After the block moves, it experiences kinetic friction, which is less than the maximum static friction.

See also

In Spanish: Fricción para niños