MR. VILLAPHYSICS

Friction

FrictionWhat is Friction?????When two surfaces are in contact, friction When two surfaces are in contact, friction

forces oppose relative motion or impending forces oppose relative motion or impending motion.motion.

Friction forcesFriction forces are are parallelparallel to the surfaces in to the surfaces in contact and contact and opposeoppose motion or impending motion or impending motion.motion.

PP

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 (mu) to determine a specific friction from a specific surface……….

22 N N

Friction and the Normal Force

The force required to overcome The force required to overcome staticstatic or or kinetic kinetic friction is proportional to the normal friction is proportional to the normal force, force, FnFn.

4 N4 N

Fk = kFnFk = kFnFs = sFnFs = sFn

FFnn12 N12 N

6 N6 N

FFnn8 N8 N

4 N4 N

FFnn

There are 2 types of Friction

Static FrictionStatic FrictionNo Relative motionStatic Friction will

resist motion and counteract any applied force up to a certain maximum where friction is overcome and motion begins.

Kinetic FrictionKinetic FrictionRelative MotionOnce an object is in

motion, kinetic friction will resist motion. This resistance is independent of velocity.

Fs = sFnFs = sFn Fk = kFnFk = kFn

Friction Continuing….

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.

44 NN 44 NN

Friction forces are independent of temperature, provided no chemical or

structural variations occur.

Heat can sometimes cause surfaces to become deformed or sticky. In such cases, temperature can be a factor.

44 NN 4 N4 N

Friction Forces

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.

2 2 NN2 2 NN

5 m/s5 m/s 20 m/s20 m/s

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

Note that the kinetic friction force remains constant even as the velocity increases.

FaFSa

NsS FF

Nkk FF

Example 1

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

Fk = kFnFk = kFnFaFk

Fn

Fg