Is a force opposing the motion of one surface over another.

walkingtires

brakes

sand on icy streets

Friction Force vs. Weight

W

Ff

Friction is caused by the microscopic roughness between surfaces…like two gears locking together.

Cause of Friction

Basically it is what slows things down… but that’s not all!

Friction can also cause heat!

Factors that affect the size & force of the Friction:

a. The roughness of the surface.

b. The amount of force pushing the surfaces together. Try it with your hands!

Which would have greater friction force… sand paper or a waxed floor?

Why?

1. Sliding Friction – acting between two surfaces where one is moving.

Pushing your binder to the middle of your desk

Ice skating

Wiping off a counter

2. Rolling Friction – acts between two surfaces of objects where one has a rounded shape.

Riding your bike

Roller skatingMoving

something with a dolly

3. Fluid Friction – opposes the motion of objects traveling through liquids or gases.

Liquids

Gases

Hammer Feather

Slipping on a wet floor

Holding your hand out of the

window of a moving car

Putting lotion on

your hands

4. Static Friction – When object is at rest, the frictional force is equal to the applied force. This static friction acts between two stationary (still) bodies . The friction is static as there is no motion. In this case, static means STILL, not moving!

Reducing the force pushing both surfaces together

Lubricants such as oil

Switch from sliding

friction to rolling friction

Polish the surfaces to make them smoother

It is the force of friction that opposes an object moving.

Many people think that it is a nuisance because it has causes us to apply a greater force to move an object.

But in fact, it is of great help to us.

If there is no friction, then cars cannot move on the road, we would have to

hold things with both hands, and we can hardly

even walk.

What if we didn’t want friction?

Sometimes we want to reduce the amount of friction and go fast!

• Think about when you have been ice skating or the sidewalk was frozen over. Is it very hard to walk on ice?

• How ‘bout those penguins?

Watch this and think about why this myth is true. Be ready to discuss.