1. The force of Friction
Chapter 4.4

2. Static Friction
The resistive force that keeps an object from moving when pushed across a surface is called static friction
When the applied force is as great as it can be without causing the object to move, the force of static friction reaches its maximum value

3. Kinetic Friction
When the applied force exceeds the maximum value for static friction, the object begins to move with an acceleration
The frictional force on an object in motion is called kinetic friction
Visual Concept

4. Which is easier? To push a chair across the floor at a constant speed
To push a heavy desk across the floor at the same speed

5. Ff proportional to FN
It is easier to push a chair across the floor at a constant speed than to push a heavy desk across the floor at the same speed.
The magnitude of the force of friction is proportional to the magnitude of the normal force
Since the desk is heavier than the chair, the desk experiences a greater normal force, therefore greater friction

6. The Coefficient of Friction
Chapter 4
The Coefficient of Friction
The quantity that expresses the dependence of frictional forces on the particular surfaces in contact is called the coefficient of friction, .
It is the ratio between friction force and normal force
It is always a number that is between 0 and 1.
It is unit less

7. Coefficient of Friction
Chapter 4
Coefficient of Friction
Related to the type of surface
Visual Concept

8. Friction Equation The “FUN” equation!! Ff = Friction Force (N)
 = coefficient of friction (no units)
FN = Normal Force* (N)
*The normal force is usually the same as an objects weight, except for when something is pulling up or pushing down on the object even though it is on a flat surface.

9. Example #1
Suppose that  between a table and the floor is If the table has a mass of 5 kg, then what is the force needed to push it so that it moves with a constant speed?

10. Example #2
A force of 240 N is used to pull a 60 kg sled along snow-covered flat ground. If the friction coefficient is 0.28, then find the acceleration.

11. Example #3
A 50 kg sled is pushed along snow-covered ground. The friction coefficient is 0.13
What force is needed to accelerate the sled at a rate of 5.6 m/s2? 
How far will it have gone after 5 seconds of pushing?

12. Practice Problems
Suppose  between a book and a tabletop is If the book’s mass is kg then what is the force needed to push it so that it moves with a constant speed?
A force of 80N accelerates a 6.0 kg block of wood at a rate of 6.6 m/s2 along a level surface. Find the coefficient of friction.
A force of 80 N accelerates a 7.0 kg box at 4.7 m/s2 along a level surface.
Find the coefficient of sliding friction for the box.
How far will the box have gone after 5 seconds of pulling it?