# Friction

When it becomes difficult for an object to freely move across a surface, friction may be the preventing force. Friction is the resistance of motion when an object rubs against and acts in the opposite direction of another object. When any two objects rub against each other they cause friction. Friction is a force that holds back another object from freely moving. A simple example are the brakes on an automobile.

Friction causes a second object to lose energy by slowing its motion. The energy does not disappear, but it changes from moving energy, which is called kinetic energy to heat energy or thermal energy. When a person rubs their hands together friction is generated and then it turns into heat. This is why cold hands become warm after rubbing them together. This is also called kinetic friction.

Friction can be found anywhere objects come into contact with each other. The brakes on a car causes friction between the brake pads and the wheels of the car, allowing the car to come to a stop. A person running on a sidewalk may stop quickly because of friction caused between the bottom of the shoes and the asphalt or concrete.

However, other variables can lessen the effects of friction. For instance, if the same person running tried to stop on a water-covered sidewalk, friction would be less and the runner may not be able to stop as quickly, and in some cases may fall. This is similar to a car trying to stop on an ice-covered roadway. The friction is still there, but it is much less and may lead to accidents. Also, during rain, there is still friction between the brakes and the wheels, yet if the brakes are wet, the wheels would not be as much in contact with the ground. As a result, cars hydroplane when they go too fast on puddles of water.

There are times when friction needs to be prevented so things will move more easily. Lubricants like grease and oil can help reduce friction between two objects. Engines and machines use grease and oil to reduce friction and wear so they can last much longer. Friction can also be reduced by using a ball or wheel on certain objects, which is called rolling friction. Changing the types of materials that come in contact with each other is another way of preventing friction. A good example is the use of roller skates on a concrete surface, ice skates on a lake covered with ice, or wearing rubber shoes on a wet sidewalk.

Besides dry friction as in some of the examples above, there is also static friction when objects are not moving such as the touching a metal surface and feeling a shock. When friction involves a fluid or air it is called fluid friction. The air resistance on an airplane, water resistance on a boat, and the slides at water parks are examples of fluid friction.

Finally, the two main factors that influence the total amount of friction include the roughness of the objects' surfaces and the force applied between the two objects. The measure of friction, its coefficient, is based on the roughness of the materials that come in contact with each other. For example, concrete on concrete has a very high coefficient of friction, and the Teflon surfaces of pots and pans have a low coefficient because it is a surface where most things do not stick.

In summary, friction is the resistance of motion when an object rubs against and acts in the opposite direction of another object. There are four types of friction which includes kinetic friction, dry friction, rolling friction, static friction, and fluid friction. Common examples of friction are brakes on a car, ice skates, and wearing rubber shoes on a wet surface. Roughness and force of objects are factors of friction, and the coefficient of friction is a measure of how easily one object moves in relationship to another.

A: Kinetic
B: Static
C: Dry
D: Thermal

A: Kinetic
B: Fluid
C: Static
D: Rolling

A: Kinetic
B: Static
C: Fluid
D: Rolling

A: A boat's surface scrapes the bottom of a body of water
B: Wheels on a car are not in contact with the roadway but moves across water
C: The air resistance felt by an airplane as it moves through the air
D: An airplane moves across the sky through a storm

A: Concrete
B: Oil
C: Grease
D: Water

A: Eggs on Teflon
B: Rubber on concrete
C: Ice skates on ice
D: Tires on water

Related Topics
Static Friction Formula
Kinetic Friction Formula
Friction Formula
Rolling Friction Examples
Stopping Distance Formula
Kinetic Friction Calculator
Static Friction Calculator
Forces and Straight-Line Motion
Formulas: Physics Formulas and Math Formulas