Unit #3 Dynamics

Dynamics – Frictionhttp://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

Unit #3 Dynamics

Sample Problem #1

Unit #3 Dynamics

Sample Problem #2Question: A box is pushed toward the right across a classroom floor. The force of friction on the box is directed toward the

1. left2. right3. ceiling4. 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

Unit #3 Dynamics

Sample Problem #3

Unit #3 Dynamics

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