1. Boy, this stuff rubs me the wrong way…
Friction
Boy, this stuff rubs me the wrong way…

2. Friction Friction is a contact force when two surfaces touch
Friction is always in the opposite direction of the motion

3. How do we get friction? The surfaces in contact with each other
Friction force depends on two things:
The surfaces in contact with each other
For example, sandpaper on wood, pavement on rubber, ice on metal
The mass of the object/surface that is trying to move
We will work with keeping one surface still and one surface moving

4. The interaction between two surfaces
The part of friction that depends on the two surfaces interacting is called the “coefficient of friction”
Abbreviated “m”
The larger the m, the more friction force there is
For example, ice and skin has a small m, sandpaper and skin has a large m

5. Just to confuse things…
There are two different types of friction:
Static Friction
The friction force that occurs between two objects as one object is trying to start moving
Kinetic Friction
The friction force that occurs between two objects once one object is moving
Typically, static friction is higher than kinetic friction (easier to keep moving than to start moving)

6. Mathematically, friction forces look like this:
The Equation
Mathematically, friction forces look like this:
Ff = (m)(FN)

7. Example #1
A person exerts a 36 N horizontal force as he pulls a 52 N sled across a cement sidewalk with constant speed. What is m between the cement and the metal sled runners?
Ff = 36 N (because motion is constant speed = no acceleration)
Ff = (m)(FN)
36 N = (m) (52 N)
.692 = m FN
36 N Ff
52 N

8. Example #2
Suppose the same sled is on packed snow. The m now equals If a person weighing 650 N sits on the sled, what force is needed to pull the sled at constant velocity?
FN = 702 N (the weights of the two objects added)
Ff = (m)(FN)
Ff = (.12) (702 N)
Ff = 84.2 N FN ? Ff
52 N
650 N

9. Friction on slanted surface
Is friction higher or lower on a slanted surface?
Well, let’s look at the free-body diagram…
Notice what happens to the normal force…
The Fg is acting straight down
Only a portion of the weight is reflected on the FN
The Ff is smaller! Ff FN Fg

10. Friction Homework Chapter 6 #33, 34, 37 #100,000,001 – 101,000,000
Every 3rd problem, except the palindromes (like 101,000,101)
Just Kidding
#33, 34, 37