Aim: How can we define friction?

Do Now:

A 20 kg object is being pulled across a table at a constant velocity with a force of 58.8 N.

Draw a free-body diagram labeling all the forces

F = 58.8 N

Fg

FN

FF

What is the weight?

= 196 N

Fg = mg

Fg = (20 kg)(9.8 m/s2)

Fg = 196 N

What is the normal force?

FN = Fg (at rest)

FN = 196 N

F = 58.8 N

Fg

FN

FF

= 196 N

= 196 N

What is the force of friction?

FF = F (constant velocity)

FF = 58.8 N

= 58.8 N

FrictionFriction

Friction occurs at the boundary of two Friction occurs at the boundary of two surfacessurfaces

No surface is perfectly smoothNo surface is perfectly smooth

FrictionFriction

The force that opposes motion measured The force that opposes motion measured in newtons (N)in newtons (N)

Always opposite direction of motionAlways opposite direction of motion

““Static Friction” is the force that opposes Static Friction” is the force that opposes the “start of motion”the “start of motion”

““Kinetic Friction” is the force of friction Kinetic Friction” is the force of friction between objects in contact that are in between objects in contact that are in motionmotion

What Friction Depends OnWhat Friction Depends On

There are two factors that affect friction:There are two factors that affect friction:The types of surfacesThe types of surfacesThe normal forceThe normal forceWhat can Mr. Foederer do to not slip as What can Mr. Foederer do to not slip as

much in the winter?much in the winter?

Coefficient of Friction

Represented as µ

FF = µFN µs = coefficient of static friction(objects at rest)

µk = coefficient of kinetic friction(objects in motion)

In the reference table

µ is a unitless number between 0 (no friction) and 1 (100% friction)

Snow and ice have a low µ

Rubber has a high µ

Low coefficient of frictionHelpful

SkiingHurtful

Driving in the snow

High coefficient of friction

Helpful

Applying the brakes to stop a car

Hurtful

Trying to push an object that has too much friction

http://www.youtube.com/watch?v=hOt-D_ee-JE

Which was harder to do? – start the object moving or keep the object moving?

Which µ is larger?

µs

µs > µk

Ex: Turning a revolving door

Harder to start it moving then to keep it moving

µ is not dependent on the surface area – only the texture

12

3 1, 2, and 3 are all the same item. Which one has a higher µ?

They all have the same µ

What is the coefficient of friction?

FF = µkFN (µk because the object is moving)

58.8 N = µk(196 N)

0.3 = µk

What are the object and table made out of?

wood and wood

A piece of copper whose mass is 10 kg is placed on a steel beam as a force is applied to it causing it to move with a constant velocity.

F

Fg

FN

FF

Draw a free-body diagram labeling all the forces

What is the weight?

Fg = mg

Fg = (10 kg)(9.8 m/s2)

Fg = 98 N

What is the normal force?

FN = Fg

FN = 98 N

What is the net force?

0 N (velocity is constant)

F

Fg

FN

FF

= 98 N

= 98 N

A force of 30 N is applied and the copper does not move. What is the force of friction?

FF = 30 N

F

Fg

FN

FF

= 98 N

= 98 N

= 30 N= 30 N

What is the maximum force that can be applied before the object begins to move?

Solve for the maximum static force of friction

FF = µsFN

FF = (0.53)(98N)FF = 51.94 N