The Force of Friction Date ________
Objects must be acted on by a force to overcome friction.
Friction is the force that opposes motion or the attempted motion of two materials or media that are touching.
Friction depends on the types of
materials and how much they are
pressed together.
The direction of the friction force is always opposite
to the motion of the object.
Free body diagram for
the block moving at a constant velocity
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When an object is at rest on a surface it is in _______________.
The net force is _________.
When a small horizontal force is applied to the object, the surface exerts an equal force in the opposite direction and the object remains at rest.
The force opposing the motion is static friction.
When the applied force exceeds the force of static friction, the object accelerates in the direction of the net force.
http://regentsprep.org/regents/physics/phys01/friction/default.htm
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As the applied force increases, so does the static friction force. When the applied force is greater than the static friction force the object moves.
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Static Friction
On the surface of all objects there are tiny bumps and
ridges that catch on each other when two objects are moving past each other.
If you try to slide the object with a small force and the
object doesn't move, the force of friction equals the applied force.
This is static friction.
Static friction depends on the type of materials in contact and the weight of the object.
The coefficient of friction is a quantity that expresses how dependant the frictional forces are on the materials in contact.
The coefficient of static friction is the ratio of maximum value of the force of static friction to the normal force.
μs = Fs,max
FN
Fs,max = μsFN
where μs is the coefficient of static friction,
Fs,max is the maximum force of static friction,
FN is the normal force
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Practice Problems
1) A 24 kg crate full of new ipods for my Physics students is initially at rest. It requires a 75 N horizontal force to set it in motion.
Find the coefficient of static friction between the crate and the floor.
We will always work on a
horizontal suface so FN = Fg
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Sliding Friction
Sliding friction occurs when there is a relative (sliding)
motion where two surfaces are in contact.
If you apply more force than the opposing maximum static friction force the object will accelerate.The force opposing the motion of the object while
it is still moving is sliding friction.
Sliding friction opposes motion less than static friction.
Sliding friction depends on the type of materials in contact and the weight of the object.
Once an object is moving the force opposing motion is the kinetic friction force NOT the static friction force anymore.
The coefficient of kinetic friction is the ratio of the kinetic force to the normal force.
μk = Fk
FN
Fk = μkFN
where μk is the coefficient of kinetic friction,
Fk is the force of kinetic friction,
FN is the normal force
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Practice Problem
A 20.0 kg box of chocolate chip cookies is pushed across the floor the Mrs. Bugosh's room by a horizontal force. If the coefficient of friction between the floor and the box is 0.255, what is the magnitude of the frictional force opposing the applied force?
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Examples of Coefficients of Friction The higher the number the more friction.
Surfaces
µ(static)
µ(kinetic)
Steel on steel
0.74
0.57
Glass on glass
0.94
0.40
Metal on Metal (lubricated)
0.15
0.06
Ice on ice
0.10
0.03
Teflon on Teflon
0.04
0.04
Tire on concrete
1.00
0.80
Tire on wet road
0.60
0.40
Tire on snow
0.30
0.20
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Interpreting free body diagrams with friction
FN = ________________
Fk = ________________
Fa = ________________
μk = 0.2
Fg = ________________
m = 10 kg
a = 0 m/s2
Fnet = _______________
What is the motion of this object?
How do you know?
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Rolling Friction
Rolling friction is the force that opposes the
motion of a round object rolled along a flat surface.
The rolling friction force is less than both the
static and sliding friction forces.
Fluid Friction (Drag Force)
Fluid friction is the force that opposes the motion of an object in a fluid (liquid or gas).
Fluid friction depends on the _____________ ______________ __________ of the object, the ________________ of the object in the fluid, the density
of the fluid and the fluid friction coefficient.
What do you feel when you stick you hand out of the window of a fast moving car?
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