1. Friction
Unit 7
Section 3

2. I. Friction
a. Friction is a force that opposes motion between two surfaces that are in contact (two surfaces rubbing together).
b. Think of friction as GRIP.
c. Friction can stop an object from moving, such as a ball rolling on the grass. The friction between the ball and the grass slows the ball and then stops it.

3. II. The Effect of Force on Friction
a. The amount of friction depends on how much force is pushing the surfaces together.
b. The heavier the objects, the more friction there is between them.

4. III. Surfaces and Friction
Rough surfaces have greater friction (grip) than smooth surfaces.
Example: A grassy field would have more friction (grip) than an ice skating rink.

5. IV. Kinetic Friction KINETIC FRICTION is “moving friction.”
i. Example- moving a piece of furniture across a carpet.

6. V. Static Friction
a. Static Friction is when a force is applied to an object, but the object does not move.
b. Static means “not moving”.
i. An example would be trying to move your bed, but it doesn’t budge. Static friction has occurred.

7. VI. Harmful Friction
a. Harmful friction is when damage is done, such as:
-when machine parts wear down
-when you get holes in your socks
-you skin your knees on the ground

8. VII. Helpful Friction
Helpful friction is when friction is helpful to us.
i. An example of helpful friction would be:
1. adding sand to make a car move in the snow
2. being able to walk without falling
3. using a pencil to write

9. VIII. Ways to Reduce Friction
Make surfaces smoother
i. For example sanding wood or polishing a floor.
2. Use lubricants, such as oil, grease, soap, wax, or water.
i. For example-putting water on a slip and slide
ii. Putting soap on a finger to take off a ring.
3. Use ball bearings or wheels to turn static friction into kinetic friction.
i. For example-putting wheels under a box to move it across a room.

10. IX. Ways to Increase Friction
Make surfaces rougher, such as adding sand to an icy road, or wearing gloves to grip a baseball bat, using a rubber grip to open a jar.
Increase the force between the surfaces, such as pushing down harder when sanding wood.