1. A force that opposes motion.
Friction
A force that opposes motion.

2. Any object has a rough surface. Some are rougher than others.
When two objects are in contact the hills and valleys on one surface stick to the hills and valleys of the other forming chemical bonds which are then broken and reformed.
The amount of friction is determined by the amount of force pushing the surfaces together and the roughness of the surfaces.
What Causes Friction?

3. The amount of friction depends on the amount of force pushing the objects together. Increasing the force increases the friction.
Changing how much of the surfaces come in contact does not change the amount of friction.
Force and Friction

4. Rougher surfaces have more hills and valleys, therefore more friction
Rougher surfaces have more hills and valleys, therefore more friction. (Try sliding a hockey puck on gravel rather than ice).
Surfaces and Friction

5. Types of Friction There are two types of friction, kinetic and static.
Kinetic friction is friction between moving surfaces. . The amount of kinetic friction is determined by how the surfaces move. You can have sliding or rolling kinetic friction. Sliding kinetic friction is used when you write or scratch yourself.
Anything that has wheels uses rolling kinetic friction.
Types of Friction

6. Forces can be applied to an object but the object doesn’t move
Forces can be applied to an object but the object doesn’t move. (Try pushing a loaded bus.) Even though the force is applied no movement occurs. This is an example of static friction. It disappears when an object starts moving and reappears when it stops.
Static Friction

7. Well, you couldn’t walk, or drive, or use brakes to stop a bike or car
Well, you couldn’t walk, or drive, or use brakes to stop a bike or car. You couldn’t write a note or draw a picture.
So, What Good Is It?

8. Why is it Harmful? It can cause holes in your pants or socks,
It can cause blisters on your hands or feet.
It causes erosion, wearing of automobile parts, and makes moving heavy items difficult.
Why is it Harmful?

9. Sometimes we want more…..
We use rough pads to clean dishes, cleats to get a better grip on the ground, sandpaper to smooth wood or metal, and brakes to slow and stop us.
Sometimes we want more…..

10. We use lubricants like oil to preserve moving parts in automobiles.
We wax skies, snowboards and surfboards in order to go faster.
We use ball bearings in skateboard and in line skate wheels to reduce friction.
We also use wax and grease to smooth surfaces.
And sometimes less…..