2. Kinetic and Static Friction
• Friction is a force that opposes the motion of two
surfaces that are in contact with each other.
• There are two types of friction—static friction and
kinetic friction.
3. Kinetic and Static Friction
• Static friction is the force exerted on one surface
by another when there is no motion between the
two surfaces.
• For example, in the image on the left, the force
applied to the couch is balanced by the static
friction force. The couch does not move.
4. Kinetic and Static Friction
• If you push hard enough, the couch will begin to
move, and kinetic friction will act on it.
• Kinetic friction is exerted on one surface by another
when the two surfaces rub against each other
because one or both of them are moving.
5. Causes Of Friction
• Matter is not as smooth on its surface as it
appears to be; tiny irregularities get “stuck” on
one another
• On very smooth surfaces, the atoms can
become so close that there are electrical
forces acting between them
• Sliding objects often experience “jerky”
motion as bonds or “jags” are broken
6. Kinetic and Static Friction
• Frictional force depends on the materials that the
surfaces are made of.
• The different lines
correspond to
dragging a block
along different
surfaces.
7. Kinetic and Static Friction
• The slope of this line is the coefficient of kinetic
friction (μk) between the two surfaces. It relates the
frictional force to the normal force, as shown below.
Kinetic Friction Force
Ff, kinetic = μkFN
• Notice that the normal force (FN) also plays a role in
the size of the friction force.
8. Kinetic and Static Friction
• Similarly, μs is the coefficient of static friction between
the two surfaces.
Static Friction Force
Ff, static = μsFN
• μsFN is the maximum static friction force that must be
overcome before motion can begin.
12. Example
Problem
A child drags a heavy, rubber-soled shoe by its laces
across a sidewalk at a constant speed of 0.35 m/s. The
shoe has a mass of 1.56 kg and the coefficient of
kinetic friction is 0.65. What is the force exerted by
the child?
13. Problem
A 15 kg box is pulled by a force of 75 N across a floor. If the box
accelerates at 3.0 m/s2, what is the coefficient of friction
between the box and the floor?
