1. Friction

2. Frictional Forces
Friction has its basis in surfaces that are not completely smooth:

3. Static Friction

4. Static Friction
Static friction keeps an object from starting to move when a force is applied.
Static frictional force has a maximum value, but may take on any value from zero to the maximum,

5. Static Frictional Forces
(6-2)
where
(6-3)

6. Kinetic Friction
Fpush > fk = mkn

7. Kinetic Friction
Kinetic friction: the friction experienced by surfaces sliding against one another
Kinetic frictional force depends on the normal force:
(6-1)
Constant is called the coefficient of kinetic friction.

8. Kinetic Friction and Speed
kinetic friction is independent of relative speed of the surfaces, and of their area of contact.

9. Coefficients of Friction
fs £ msFn
fs max = msFn
fk = mkFn
Typically, ms>mk

10. Horizontal Retarding Friction
A cruise-ship passenger uses a shuffleboard cue to push a shuffleboard disk of mass 0.40 kg horizontally along the deck, so that the disk leaves the cue at a speed of 8.5 m/s. The disk then slides a distance of 8.0 m.
What is the coefficient of kinetic friction between the disk and deck?
m = 0.40 kg
v0x = 8.5 m/s
x0 = 0
x = 8.0 m
vx = 0

11. Friction ACT1
You push two identical bricks across a tabletop with constant speed v. Which Case, as shown above, requires more force Fn?
a. F1>F2 b. F1=F2 c. F1<F2 d. Need to know brick areas to answer

12. Inclined Plane Friction
Suppose a 20 kg object is sliding down a 50o inclined plane. If µk = 0.4 between the plane surface and the object, what is the object’s acceleration down the plane? How long will it take the object to slide 5.0 m if it starts from rest?. k F

13. Friction with  Component of Tension
Two children sitting on a sled at rest in the snow ask you to pull them. You oblige by pulling on the sled’s rope, which makes an angle of 40° with the horizontal. The children have a combined mass of 45 kg, and the sled has a mass of 5.0 kg. The coefficient of kinetic friction is mk=0.15. Find the acceleration of the sled and children if the rope tension is 140 N.

14. Antilock Brakes

15. Antilock Brakes
A car is traveling at 30 m/s along a horizontal road. The coefficients of friction are ms=0.50 and mk=0.40.
(a) What is the braking distance Dxa with antilock brakes?
(b) What is the braking distance Dxb if the brakes lock?