Friction is the force that resists the relative motion or tendency to move between two surfaces in contact. It acts opposite to the direction of motion or attempted motion, thereby slowing down or preventing movement
. Friction occurs between solid surfaces, fluid layers, or material elements sliding against each other, and it arises mainly due to microscopic irregularities on surfaces and adhesive forces at contact points
. The rougher the surfaces, the greater the friction produced
. There are different types of friction, including dry friction (between solid surfaces), fluid friction (between layers of fluids), lubricated friction, skin friction, and internal friction within materials
. Key characteristics of friction include:
- It is nearly independent of the contact area between surfaces
- It is proportional to the normal load pressing the surfaces together, with the ratio of frictional force to load called the coefficient of friction (μ), a dimensionless number
- Friction converts some mechanical energy into heat and other forms of energy dissipation
Friction has practical effects such as enabling walking without slipping, preventing tires from skidding, and even starting fires by rubbing materials together
. However, it also causes energy losses, accounting for about 20% of the world's energy expenditure
. In summary, friction is a fundamental force in everyday life that resists motion between surfaces, dependent on surface texture and load, and plays both beneficial and limiting roles in mechanical systems.
