1. Friction Notes
Friday, 9/30 and Monday, 10/3

2. What is friction? Occurs when two objects are pressed together.
The contact causes a frictional force between the surfaces of the objects.
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Image source:
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3. What is friction? Is parallel to the surface.
Opposes the relative motion of the two surfaces.

4. What is the difference between Kinetic and Static Friction
Kinetic friction – when the two surfaces are moving relative to each other (ex: the wheels of the wagon).
Kinetic friction is dependent on
Normal force between the surfaces Fn
The nature of the surfaces
Each surface is unique (driving on gravel vs. driving on a hockey ice).
The uniqueness of surfaces will dictate the frictional force. In Physics we use the coefficient of friction to differentiate between surfaces.
Coefficient of friction (μk)
Frictional force is = to product of the normal force and the coefficient of friction. Expressed as
Ff,k = μkFn

5. What is the difference between Kinetic and Static Friction
Static friction – when the two surfaces are at rest relative to each other (ex: cat sitting on top of the book).
Static friction is dependent on
The forces acting on the object
The static friction will have a large magnitude because objects resist change (inertia).
Coefficient of friction (μs)

6. Maximum static friction
The maximum force of static friction is dependent on:
Normal force between the surfaces (Fn)
Nature of the surfaces coefficient of friction (μs). (note this is different than coefficient of μk.
Ff,s,max = μsFn
0 ≤ Ff,s, ≤ μsFn
Static friction is stronger than kinetic friction μs >μk

7. accelerating to the right
In a truck bed
accelerating to the right
Sliding Down
an Incline
Box sliding to right

8. A 4 kg box slides to the right on a level surface
A 4 kg box slides to the right on a level surface. The coefficient of kinetic friction between the box and the surface is What is the force of kinetic friction?

9. A 10 kg box is at rest on a surface which is inclined at a 30° relative to the horizontal. What is the frictional force on the box?