Download Friction

Friction Notes
Friday, 9/30 and Monday,
10/3
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: http://www.aplusphysics.com/courses/honors/dynami
cs/images/timbo_
pulling_traci_in_a_wagon_hg_wht.gif
What is friction?
 Is
parallel to the surface.
 Opposes the relative motion of the two
surfaces.
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
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)
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
Box sliding to right
Sliding Down
an Incline
In a truck bed
accelerating to the
right
A 4 kg box slides to the right on a level surface. The
coefficient of kinetic friction between the box and
the surface is 0.50. What is the force of kinetic
friction?
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?