# what is friction?

boblawrence | Certified Educator

Friction is defined as the force resisting the relative motion of surfaces or fluid layers sliding against one another.

Types of friction and examples are as follows:

Dry Friction:  Friction between two solid surfaces sliding over each other.

Example:  automobile disk brakes

Lubricated Friction:  Friction when the surfaces are separated by fluid.

Example:  Hockey puck on melting ice rink

Fluid Friction:  Friction between layers in a viscous fluid.

Example:  Slow oozing of a highly viscous fluid such as volcanic lava, compared to rapid movement of a low viscosity fluid such as water.

Skin Friction:  The force equivalent to the drag that impends movement of a solid through a liquid

Example:  Space shuttle re-entry into earth’s atmosphere

The classical description and basic rules of friction were discovered by Leonardo da Vinci (1452-1519).

When objects move against one another their kinetic energy is converted to heat, the primary manifestation of dry friction.

Friction can also wear away and damage surfaces.

Friction is not a fundamental force.  Instead, it is a result of complex interactions at the molecular level on the surfaces involved.  Because of this complexity and the extreme variety of surface components, friction cannot be calculated by formula. It must be measured empirically through experimentation.

In addition to details of information given herein, the reference describes the laws of the friction types.  It also defines and explains the coefficient of friction, a useful engineering concept that expresses the ratio of friction between moving surfaces to the force pressing the surfaces together.

sid-sarfraz | Student

Friction is the opposing force that tends to stop the object we are trying to move in a certain direction. Friction basically depends on the nature of the surface and the amount of force applied or the weight of the object on the surface. Friction can be static, sliding, rolling and air resistance. Friction produces heat.