Physics

4.3 - Friction

2 min

note 1 of 2

Friction

Friction is a force that opposes motion.
It always acts in the opposite direction to the way an object is moving, or trying to move.
Without a force pushing it along, a moving object gradually slows down and stops because of friction.
To keep moving at a steady speed, an object needs a driving force (such as thrust from an engine) that balances the frictional force, and if the driving force is larger, as in the car below, the object speeds up.

note 2 of 2

Where friction occurs

Between two solid surfaces in contact - both when surfaces grip and aren't moving (static friction) and when they slide past each other (sliding friction).
When an object moves through a fluid (a liquid or a gas). Friction in a fluid is called drag, and air resistance is a type of drag.

Reducing friction between solids

Friction between solid surfaces can be reduced using a lubricant, such as oil or grease.
Friction can also cause solid surfaces to wear away over time.

Friction causes heating

Friction transfers kinetic energy into thermal energy, causing the surfaces and surroundings to heat up.

key terms

TermDefinition

FrictionA force that opposes motion, acting in the opposite direction to the way an object is moving or trying to move.

DragThe frictional force that acts on an object moving through a fluid.

Air resistanceA type of drag that acts on objects moving through the air.

LubricantA substance, such as oil or grease, placed between surfaces to reduce friction.

end of notes

Key points to remember

Friction

Friction is a force that opposes motion.

It always acts in the opposite direction to the way an object is moving, or trying to move.

Without a force pushing it along, a moving object gradually slows down and stops because of friction.

To keep moving at a steady speed, an object needs a driving force (such as thrust from an engine) that balances the frictional force, and if the driving force is larger, as in the car below, the object speeds up.

Where friction occurs

Between two solid surfaces in contact - both when surfaces grip and aren't moving (static friction) and when they slide past each other (sliding friction).

When an object moves through a fluid (a liquid or a gas). Friction in a fluid is called drag, and air resistance is a type of drag.

Reducing friction between solids

Friction between solid surfaces can be reduced using a lubricant, such as oil or grease.

Friction can also cause solid surfaces to wear away over time.

Friction causes heating

Friction transfers kinetic energy into thermal energy, causing the surfaces and surroundings to heat up.