1. A Force that Opposes Motion
Section 1.3: Friction
A Force that Opposes Motion

2. The Source of Friction
The amount of friction depends on many factors, which is the force pushing the surfaces together & the roughness of the surfaces.
When two surfaces are in contact, the hills & valleys of the surfaces “stick” together

3. The Effect of force on Friction
Amount of friction depends on the force pushing the surfaces together
Objects that weigh less, exert less downward force
Changing the amount of surface contact does NOT change the amount of friction

4. The Effect of Rougher surfaces on Friction
Rough surfaces have more microscopic hills and valleys than smooth surfaces
Friction is greater for rougher surfaces

5. Kinetic Friction Friction between moving surfaces
Surfaces can slide past each other or a surface can roll over another surface

6. Sliding Kinetic Friction
Usually a greater force than rolling kinetic friction
Examples: applying brakes on a bike, writing with a pencil, scratching an itch

7. Rolling Kinetic Friction
It is easier to move objects on wheels, than by sliding
Important part of almost all means of transportation
examples: bicycles, skating, trains, cars, etc.

8. Static Friction Static means not moving
When a force is applied to an object, but the object does not move, static friction occurs
Static friction disappears as soon as an object starts moving
Kinetic friction immediately occurs
Applying a large enough force can overcome static friction

9. Helpful Car tires could not push against the ground to move the car
Brakes could not stop the car
You would slip and fall when you tried to walk

10. Harmful Moving parts in a car’s engine wear down
Wind and water can cause erosion of soil
Socks and jeans develop holes (wear out)

11. Reducing Friction Use lubricants between surfaces (oil, grease, wax)
Switch from sliding kinetic friction to rolling kinetic friction
Make surfaces that rub against each other smoother

12. Increasing Friction Make surfaces rougher
Example: sand on icy roads
Increase the force pushing surfaces together
Example: pressing harder when sanding wood