httpwwwaplusphysicscomcourseshonorsdynamicsfrictionhtml Unit 3 Dynamics Objectives and Learning Targets Define friction and distinguish between static and kinetic friction Determine the coefficient of friction for two surfaces ID: 618047
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Slide1
Dynamics – Friction
http://www.aplusphysics.com/courses/honors/dynamics/friction.html
Unit #3 Dynamics Slide2
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 Slide3
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 Slide4
Sample Problem #1
Unit #3 Dynamics Slide5
Sample Problem #1
Unit #3 Dynamics Slide6
Sample Problem #2
Unit #3 Dynamics
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.Slide7
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 (µ
s) for objects which are not sliding, and the kinetic coefficient (µk) for objects which are sliding.
Unit #3 Dynamics Slide8
Coefficient of Friction (μ
)Unit #3 Dynamics
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.
μ
k
≤
μ
sSlide9
Choosing μ
A sled sliding down a snowy hill.
μk
A refrigerator at rest that you want to move.
μ
s
A car with tires rolling freely. μs , Why?A car skidding across pavement.
μ
k
, Why?
Unit #3 Dynamics Slide10
Sample Problem #3
Unit #3 Dynamics Slide11
Sample Problem #3
Unit #3 Dynamics Slide12
Friction Formula
Unit #3 Dynamics
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 (F
N), therefore, can be determined using the formula:Slide13
Problem Solving
Unit #3 Dynamics
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
:Slide14
Sample Problem #4
Unit #3 Dynamics Slide15
Sample Problem #4
Unit #3 Dynamics Slide16
Sample Problem #5
Unit #3 Dynamics
http://
www.aplusphysics.com
/courses/honors/dynamics/
friction.htmlSlide17
Sample Problem #6
Unit #3 Dynamics
http://
www.aplusphysics.com
/courses/honors/dynamics/
friction.html