1. Friction
Mr. Villa
PHYSICS

2. Friction What is Friction?????
When two surfaces are in contact, friction forces oppose relative motion or impending motion.
Friction forces are parallel to the surfaces in contact and oppose motion or impending motion. P

3. Coefficient of Friction
Every surface has different degrees of friction
These different surfaces have a measurement of “how much” friction they have, we use the symbol m (mu) to determine a specific friction from a specific surface……….

4. Friction and the Normal Force
The force required to overcome static or kinetic friction is proportional to the normal force, Fn.
12 N
6 N Fn
8 N
4 N Fn Fn
4 N
2 N
Fs = msFn
Fk = mkFn

5. There are 2 types of Friction
Static Friction
No Relative motion
Static Friction will resist motion and counteract any applied force up to a certain maximum where friction is overcome and motion begins.
Kinetic Friction
Relative Motion
Once an object is in motion, kinetic friction will resist motion. This resistance is independent of velocity.
Fs = msFn
Fk = mkFn

6. Friction Continuing….

7. Friction forces are independent of area
If the total mass pulled is constant, the same force (4 N) is required to overcome friction even with twice the area of contact.
For this to be true, it is essential that ALL other variables be rigidly controlled.
4 N
4 N

8. Friction Forces
Heat can sometimes cause surfaces to become deformed or sticky. In such cases, temperature can be a factor.
Friction forces are independent of temperature, provided no chemical or structural variations occur.
4 N
4 N

9. Friction forces are independent of speed.
The force of kinetic friction is the same at 5 m/s as it is for 20 m/s. Again, we must assume that there are no chemical or mechanical changes due to speed.
20 m/s
5 m/s
2 N
2 N

10. Friction and Acceleration
When Fa is greater than static friction, the resultant force produces acceleration. However, as soon as the object begins moving, friction decreases to the kinetic frictional force a Fa FS
Note that the kinetic friction force remains constant even as the velocity increases.

11. Example 1 Fk = mkFn Fk =(.10)(5) Fk =.5 Fn Fk Fa Fg
A hockey puck has a coefficient of friction of μk = .10. If the puck feels a normal force (Fn) of 5 N, what is the frictional force that acts on the puck?
Fk =(.10)(5)
Fk =.5 Fn
Fk = mkFn Fa Fk Fg