Dynamics – Friction
http://www.aplusphysics.com/courses/honors/dynamics/friction.html
Unit #3 Dynamics

Objectives and Learning Targets
 Define friction and distinguish between static and
kinetic friction.
 Determine the coefficient of friction for two surfaces.
 Resolve a vector into perpendicular components: both
graphically and
algebraically.
 Use vector diagrams to analyze mechanical systems
(equilibrium and non-equilibrium).
Unit #3 Dynamics
Types of Friction

Friction - a force that opposes motion, acts parallel to a surface opposing the
linear velocity of an object

There are two main types of friction.
Kinetic friction is a frictional force that opposes motion for an object which
is sliding along another surface.
Static friction, on the other hand, acts on an object that isn't sliding. If you
push on your textbook, but not so hard that it slides along your desk, static
friction is opposing your applied force on the book, leaving the book in static
equilibrium.

The magnitude of the frictional force depends upon two factors:
1) The nature of the surfaces in contact.
2) The normal force acting on the object (FN).
Unit #3 Dynamics
Sample Problem #1
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Sample Problem #1
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Sample Problem #2
Question: A box is pushed toward the right across a classroom
floor. The force of friction on the box is directed toward the
1. left
2. right
3. ceiling
4. floor
Answer: (1) left. Friction opposes motion.
Unit #3 Dynamics
Coefficient of Friction (μ)
 The ratio of the frictional force and the normal force
provides us with the coefficient of friction (µ), a
proportionality constant that is specific to the two
materials in contact. You can look up the coefficient
of friction for various surfaces from a variety of
references. Make sure you choose the appropriate
coefficient... use the static coefficient (µs) for objects
which are not sliding, and the kinetic coefficient (µk)
for objects which are sliding.
Unit #3 Dynamics
Coefficient of Friction (μ)
Generally, static coefficients of
friction are larger than kinetic
coefficients of friction for the
same two surfaces. In some rare
circumstances, the two
coefficients can be equal. Never
can the kinetic coefficient of
friction be larger than the static
coefficient. Often the
coefficients of friction are
between 0 and 1.
μk ≤ μs
Unit #3 Dynamics
Choosing μ
 A sled sliding down a snowy hill.
μk

A refrigerator at rest that you want to move.
μs

A car with tires rolling freely.
μs , Why?
 A car skidding across pavement.
μk , Why?
Unit #3 Dynamics
Sample Problem #3
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Sample Problem #3
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Friction Formula
• The normal force always acts perpendicular to a surface, and
comes from the interaction between atoms that act to maintain
its shape. In many cases, it can be thought of as the elastic force
trying to keep a flat surface flat (instead of bowed). We'll use the
normal force to help us calculate the magnitude of the frictional
force.
• The force of friction, depending only upon the nature of the
surfaces in contact (µ) and the magnitude of the normal force
(FN), therefore, can be determined using the formula:
Unit #3 Dynamics
Problem Solving
• Solving problems involving friction requires us to apply the same
basic principles we've been talking about throughout the
dynamics unit...
1) drawing a free body diagram
2)applying Newton's 2nd Law along the x- and/or y-axes
3) solving for our unknowns.
• The only new skill is drawing the frictional force on the free body
diagram, and using the relationship between the force of friction
and the normal force () to help us solve for our unknowns.
Let's take a look at a sample problem:
Unit #3 Dynamics
Sample Problem #4
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Sample Problem #4
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Sample Problem #5
http://www.aplusphysics.com/courses/honors
/dynamics/friction.html
Unit #3 Dynamics
Sample Problem #6
http://www.aplusphysics.com/courses/honors
/dynamics/friction.html
Unit #3 Dynamics