Friction
• Friction is a force that opposes the motion, or
tendency of motion, of an object.
• Friction is caused mostly by the electromagnetic
interactions of particles within molecules at the
surfaces of objects in contact. While these
interactions can be rather complex, their
combined effects can be reasonably
approximated with a decent amount of
consistency.
Basic Types of “dry” Friction
– Static friction
• exists between the surfaces of non-moving objects
• Maximum static friction refers to the most force that
can be applied before the object starts to move
– Kinetic friction
• Exists between the surfaces of objects when there is
relative motion between the objects
– Rolling friction
Coefficient of Friction
• The coefficient of friction is the ratio of the
magnitudes of frictional force to the normal
force acting between two surfaces.
μ = f/FN
• Since this is a ratio of force to force, there are
no units for the coefficient of friction
• This is an experimentally determined value for
any two surface combinations. Its value
depends on the nature of the materials and is
roughly independent of surface area or speed
Coefficient of Friction
• The coefficient for static friction (μs) is generally
larger than that of kinetic friction (μk) between
surfaces.
• A common substitution to be made in problem
solving will be f = μFN.
– If working with static friction, this equation represents a
maximum possible value.
Example – kinetic, constant speed
• The coefficient of friction between a 12 kg
wooden crate and the floor is 0.32. How
much force is needed to push this crate across
the floor at a constant speed?
Example - static
• A 24 kg crate initially at rest on a horizontal
floor requires 75 N of horizontal force to set it
in motion. Find the coefficient of static
friction between the crate and the floor.
Example – accelerated motion
• A 5.0 kg box is pushed horizontally across the
floor with a force of 25.0 N. If the coefficient
of kinetic friction is 0.24, what is the
acceleration of the box?
When the applied force is acting at an angle…
• Remember, a component of that force acts vertically
and a component acts horizontally
Fa
θ
Fay = Fasinθ
Fax = Facosθ
• An upwards component will tend to separate
surfaces reducing the normal force and thus reducing
frictional force since f = μFN
• What do you think would happen if there was a
downward component to the applied force?
Example – applied force at an angle
• A 15 kg sled is dragged across the level snow
at a constant speed by a force of 40.0 N that is
applied 28° above the horizontal. Determine
the coefficient of friction between the sled
and snow.