Friction

What Is Friction

• Friction is a force that opposes motion between two surfaces that are touching.

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The Source of Friction

• Occurs because the surface of any object is rough.• Objects that feel or look smooth are actually covered

with microscopic hills and valleys.• The amount of friction between two surfaces

depends on many factors, including the roughness of the surfaces and the force pushing the surfaces together.

What Creates More Friction?

1. Rougher surfaces create more friction.

ex. Pavement vs. Grass vs. Ice

2. Greater forces create more friction.

* Depends on amount of force pushing

the two surface together.

ex. Science book on a table vs. a

notebook on a table

Types of Friction

1. Sliding Friction

2. Rolling Friction

3. Fluid Friction

4. Static Friction

Sliding Friction

• Sliding friction is very effective at opposing the movement of objects and is the force that causes objects to stop moving.

• Examples: pushing pencil across table, moving furniture, sledding, apply brakes to bike or a car, write with a piece of chalk.

Rolling Friction• The force of rolling friction is smaller

and easier to overcome than sliding friction.

• Rolling friction is an important part of almost all means of transportation.

• Examples – bikes, cars, trains, skateboards

Fluid Friction

• Fluid Friction is less than sliding frictions.• The term fluid includes liquids, such as

water and milk, and gases, such as air and helium.

• Fluid friction opposes the motion of objects traveling through a fluid.

• Fluid friction between air and a fast moving car is the largest force opposing the motion of the car.

Static Friction

• When a forces is applied to an object but does not cause the object to move, static friction occurs.

• The object does not move because the force of static friction balances the force applied.

• Static friction disappears as soon as an object starts moving, and then another type of friction immediately occurs.

How is Friction Harmful

Harmful• Friction by wind and water can cause erosion.• Friction between problems in the moving parts of

an engine…rub together and increase temperature.

Helpful• Need friction between paper and pencil to write.• Need friction to walk, without it you would just

slip and fall.

Ways to increase and decrease friction

Increase Friction• Make surfaces rougher.• Increase the force pushing the surfaces

together.

Decrease Friction• Use lubricants (substances that are applied to

surfaces to reduce friction between them.)• Switch from sliding friction to rolling friction.