Friction is a resistance to motion that is opposite the direction of travel. It is caused when two surfaces are in contact. Since friction acts to slow down a moving object, it is called a resistive force. This is different than active forces that cause objects to accelerate or change direction.

Friction is mainly caused by surface roughness. The classic law of friction states that friction is the product of a coefficient and a force. The main two types of friction--static and kinetic--create different amounts of resistance for the same objects.

Questions you may have about friction include:

bulletWhat causes friction?
bulletWhat are the different types of friction?

This lesson will answer those questions. There is a mini-quiz at the end of the lesson.

Causes of friction

Friction is caused by the roughness of the materials rubbing against each other, deformations in the materials, and a molecular attraction between materials.

Surfaces not completely smooth

Most friction results because the surfaces of materials being rubbed together are not completely smooth. If you looked at what seems to be a smooth surface under a microscope, you would see bumps, hills and valleys that would interfere with sliding motion. Of course, the rougher the surface, the more the friction.

If both surfaces become ultra-smooth and flat, the friction from surface roughness becomes negligible, but then friction from molecular attraction comes into play, often becoming greater than the normal friction.


Soft materials will deform when under pressure. This also increases the resistance to motion. For example, when you stand on a rug, you sink in slightly, which causes resistance when you try to drag your feet along the rug's surface. Another example is how rubber tires flatten out at the area on contact with the road.

When materials deform, you must "plow" through to move, thus creating a resistive force.

Molecular attraction

There is another factor in friction, and that is stickiness caused by molecular attraction. This was mentioned above where surfaces are so smooth that the materials stick together due to molecular forces.

Soft rubber is an example of a material that can have this type of friction. This factor is usually seen in rolling friction. The stickiness will create a resistance to any motion. Although this force is the smallest, it still can be a factor when the other causes of friction are low.

Types of friction

There are two main types of friction: static and kinetic. Static friction is the amount of resistance to motion when an object is static or not moving. Kinetic friction is the resistance on a moving object.

Kinetic friction is further classified as sliding or rolling friction. Kinetic friction plays a small part in air or water resistance.

Static friction

Did you ever notice how hard is is to move a heavy object, but once you start it sliding, it seems to move easier? It acts like it is almost stuck to to floor until you get it moving.

Although you need to overcome the inertia of the heavy object, much of the effort needed is due to the fact that static friction is usually greater than sliding or kinetic friction. Static friction is caused by a combination of the resistance due to the roughness of the surface and the molecular attractions trying to hold the surfaces together.

Kinetic friction

Once an object is moving, the amount of friction is less than when it was standing still. Sliding friction is a form of kinetic friction. Most of the friction is cause by the roughness of the two surfaces.

Measuring Friction

Just as a scale can be used to measure the weight of an object, it can also be used to measure the force required to overcome friction. You can place an object such as a book on a table and measure the force required to move that book.

Double the weight

Then, if you double the weight--like using two books--the force required to move the books will also double.

(Note that the push or pull must be parallel to the ground. If you pull up slightly when trying to move the objects, the force between the surfaces is reduced and the measurement is not correct.)

Two books create twice the frictional force

Two books require twice the force to pull them than one book

So, friction is an important resistive force that will slow down and stop objects from moving.

Mini Quiz