1. Do Now:
What are the units for the coefficient of friction and it’s symbol?
What affects the coefficient of friction?
Draw the FBD for a block being pulled across a table with friction.

2. Aim: How can we apply concepts of friction to real life experiences?
HW:

4. Calculating Frictional Force (FF)
Force of Friction depends on two things:
Coefficient of friction (μ)
-Static (not moving) or kinetic (moving)
Normal Force
-Big Normal force means big FF

5. What does frictional force (FF) depend on???
1. Coefficient of friction.
2. The normal force of the object to be moved.

6. Coefficient of Friction
BE CAREFUL!
Pay attention to the MATERIALS in the problem and if the object is MOVING or STATIONARY!
Different materials have different coefficients of friction.

7. 4 Steps for Newton’s Law Problem
Draw a Free Body Diagram
Sum up vertical and horizontal forces SEPERATELY.
Write Newton’s Second Law for vertical and horizontal!
Solve for Unknown

8. How much force is needed in order to get it moving?
Example #1
A wooden 15 kilogram block is motionless on a wooden table.
A force of 100 Newtons is now applied to the block. Calculate the kinetic friction and the acceleration of the block
How much force is needed in order to get it moving?
Fnet = ΣFX
Fnet = FA + Ff
Fnet = 100 N – 44.1 N
Fnet = 55.9 N
a = Fnet / m
a = 55.9 N / 15 kg
a = 3.73 m/s2
Ff = μKFN
Ff = (0.30)(15 kg)(9.81 m/s2)
Ff = 44.1 N
Ff = μsFN
Ff = (0.42)(15 kg)(9.81 m/s2)
Ff = 61.8 N

9. Example 2
A 60 kg skier is sliding along a horizontal patch of snow with constant speed. Look up μk. Draw a FBD! How much force must be applied to keep the skier moving with this constant velocity?

10. Conceptual Questions
Will μ change if I increase or decrease the mass of my object?
Does μ change if I increase the objects surface area?
Does μ change if the material in contact changes?
μ depends only on different materials in contact! FACT OF PHYSICS!
Have students close their eyes and answer!

11. Station Activity
Go to the assigned station and attempt the following problems.

12. How much force does it take to keep a 1000 kg car moving at constant speed on a dry asphalt road?
Ff = µFN = µmg
= (0.67) (1000 kg) (9.8 m/s2)
= 6,566 Newtons

13. What is the coefficient of friction between a 5 kg block of wood and sandpaper if it takes 12 Newtons of force to move the wood on the sandpaper?
Ff = µFN = µmg
12 N = µ (5 kg)(9.8 m/s2)
12 N = µ (49 N)
µ = 12 N / 49 N = .245

14. Summary
1. What does μ depend on? 2. If the Normal Force increases, what happens to the frictional force? Increase, decrease or stay the same? 3. What is the coefficient of friction for a piece of copper sitting on top of steel?