Friction

The force between two surfaces in contact that causes a resistance of the motion or

pending motion between the surfaces.

Friction always opposes the direction of motion or pending motion between the two surfaces in contact.

Applied forcefriction

Motion direction

•Friction is always parallel to the two surfaces in contact.

Friction always opposes the direction of motion or pending motion.

Applied force friction

Motion Direction

What Causes Friction?

• Irregularities between two surfaces in contact.

A microscopic viewof the two surfacesin contact.

Friction

• Measured in Newtons (N) because it is a type of force.

• Frictionless objects are fictitious. • Static friction – the type of friction present when

two surfaces are at rest with respect to each other

• Kinetic (Sliding) friction – the type of friction present when one surface slides across another surface.

• Static friction is usually greater than kinetic friction

Friction is only affected by:

• The texture or surface properties between the two surfaces in contact. (Coefficient of friction)

• The amount of contact between the two surfaces. (Normal force)

μηf

f = frictional forceμ = coefficient of frictionη = normal force

Friction equation:

μηf Friction is fun

Coefficient of friction• Surface properties between the two surfaces in contact that

affects pending motion or motion• The larger the value the more resistance between the two

surfaces.• The coefficient of static friction only applies when an object is

not sliding over another surface• The coefficient of sliding (kinetic friction) only applies when

one object slides over the surface of another surface.• The coefficient of static friction is usually greater than the

coefficient of sliding friction.• Dimensionless quantity• μ = The Greek Letter “mu”• The ratio to the frictional force to the normal force• μ = 0 for a frictionless surface.• Value range typically between 0.03 (smooth ice on smooth

ice) 1.20 (rubber on concrete)• The value for coefficient can be interpreted as a percent. The

force needed to move ice on ice is only 3 % of the normal force.

• Table

Static Friction increases to a maximum level

• Static friction matches the applied force to a certain level. After the maximum static friction value is obtained the object moves and kinetic friction is active.Applied force

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Stat

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Kineticfriction

Normal force• Represents the amount of contact

between two surfaces• Measured in Newtons.• Always directed away from the surface

and perpendicular to the two surfaces in contact.

η

η

η

Two types of friction

• Since there are two types of coefficient of frictions (static and kinetic), there are two types of friction

• fs=static friction (the friction present when one object does not slide over the surface of another surface

• fk = kinetic friction (the friction present when an object slides over the surface of another surface.)

• fs=μsη

• fk=μkη

If the applied force and frictional force are equal, then the object will move

at constant velocity

Applied force = FAFriction =f

Motion direction

FA=fs for an object not in motion

FA=fk for an object in motion that slides over the surface

of anther surface