Friction
• Friction is a force that opposes the motion, or
tendency of motion, of an object.
• Friction is primarily caused by the chemical
bonding of molecules at the surfaces of
objects in contact (intermolecular forces
which are electrical in nature)
2 Types of “dry” Friction
– Static friction
• exists between the surfaces of non-moving objects
• Opposes applied forces that would otherwise cause
motion
• 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
Coefficient of Friction
• The coefficient of friction is the ratio of the force of
friction to the normal force acting between two
surfaces.
• 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 types of surface combinations
• The coefficient for static friction is generally larger
than that of kinetic friction between surfaces.
Coefficient of Static Friction
Fs – Static friction force
Fn = Normal force
Coefficient of Kinetic Friction
Fk – Kinetic friction force
Fn – Normal force
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 – 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 – 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
When the applied force is acting at an angle…
• A downwards component will tend to squeeze
surfaces together increasing the normal force
and thus increasing the amount of frictional
force since f = μFN
• When there is a vertical component, it is most
likely that the normal force does NOT equal
the weight
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.