Force of Friction

Forces such as gravity and friction can affect the speed and direction of an object.

Force of Friction

Friction is a force that occurs when one object rubs against another object causing resistance to motion.

The force of friction resists motion.

Factors that Determine the Force of Friction:

(1) the kinds of surfaces interacting.
(2) the amount force pressing the surfaces together.

Friction is a Force that Resists Motion

Friction is the force that acts to resist sliding between two surfaces that are touching. It can slow down or even stop the motion of an object.

Interactive Guide:

What happens to the particles when the two surfaces rub together?
What happens to the temperature when the two surfaces rub together?

The force of friction always slows down or stops the motion of an object in the opposite direction.

Notice the force of friction works in the opposite direction as the object's motion.

For example, friction slows or stops the motion of moving parts of machines.

Another example would be athletic shoes with tread grooves to increase friction have better traction for starting or stopping motion than smooth-soled shoes.

Friction can also be the force that makes it difficult to start an object moving. Enough force must be applied to a non-moving object to overcome the friction between the touching surfaces.

The smoother the two surfaces are, the less friction there is between them; therefore, the moving object will not slow down as quickly.

Friction between surfaces can be reduced, in order for objects to move more easily, by smoothing the surfaces, using wheels or rollers between the surfaces, or lubricating/oiling the surfaces.

If friction could be removed, objects would continue to move forever and ever!

Types of Friction

Kinetic friction refers to the resistance created by two objects sliding against each other as the object(s) in motion. This can also be called sliding friction.

Static friction is a force that resists an applied force to an object, and the object remains at rest until the force of static friction is overcome. Kinetic friction is different because the object is moving even though the frictional force is resisting its motion. Static friction the object is unable to overcome the friction and is not moving.

Kinetic friction occurs when the applied force is large enough to overcome the static friction.

Rolling friction is the resistive force that slows down the motion of a rolling ball or wheel. It is also called rolling resistance.

Fluid friction is the force that resists motion in liquids or gases. If you've ever tried to push your open hand through the water in a tub or pool, then you've experienced fluid friction.

The Greater the Force the Greater the Friction

The greater the force pushing the two surfaces together, the stronger friction prevents the surfaces from moving.

As an object gets heavier, the force of friction between the surfaces becomes greater.
To move a heavy object, a greater force must be applied to overcome the friction between the surfaces.

Select Friction below to experiment with the force of friction

Interactive Guide:

What happens to the force of friction when the applied force increases?
What happens to the speed of the object when the friction is none?
What happens to the speed of the object when the friction is "lots?"
What direction is the force of friction arrow always pointing compared to the applied force?
What happens to the force of friction when you add more mass?