Unit Two: DynamicsPart 3: Friction

What is the symbol for weight? What is the symbol for mass?

What type of frame of reference??? You are standing in an elevator waiting for it

to go up 10 flights.

You are standing in an elevator that is just starting to move.

You are standing in an elevator going down at a constant speed.

Friction – Quick Review A contact force

Electromagnetic Force (between surface atoms of objects touching)

Always opposes motion

Friction Summary Slide There are 2 types of friction:

Static Frictional ForceWhen you start to move an object from restLarger than Kinetic Frictional Force due to

InertiaSymbol: ųs

Kinetic Frictional ForceExists when the object is movingSymbol: ųK

Why? Think about moving a large couch across the

floor. Which part is more difficult – starting the movement or continuing the movement? Why?

Starting the movement is more difficult because … (can you complete this sentence?)

Coefficient of Friction Summary “Stickiness value” ų (symbol “mu”) has no units

Page 140, table 4.5

Formula: Ff = ųFN

*** NOTE: FN = - Fg in most of our grade 11 questions (as we typically deal with forces in 1 dimension only, no angles).

Friction Example During the winter, owners of pickup trucks

often place sandbags in the rear of their vehicles. Calculate the increased static force of friction between the rubber tires and wet concrete resulting from the addition of 200. kg of sandbags in the back of the truck.

Use the table of coefficients of friction on page 140.

Friction Example 2 A horizontal force of 85N is required to pull a

child in a sled at constant speed over dry snow to overcome the force of friction. The child and sled have a combined mass of 52kg. Calculate the coefficient of kinetic friction between the sled and the snow.

Friction Example 3 A 7.0kg box is being pushed horizontally at a

constant speed. If the coefficient of friction is 0.30, how much force is being used to push the box? DRAW A FBD!

Example 4 You are pushing horizontally on a book

against a wall so that it does not slide down the wall. Draw a FBD for this situation. What condition is

necessary for the box to not slide down the wall? If you lessen the horizontal push that you are

exerting, the box will start to slide down the wall. Explain why this happens.

Practice Friction Problems Page 144 Questions 5, 6, 7, 8

LAB!!!

Day 2 Go through errors!

Check Your Learning A friend pushes a 625g textbook horizontally along a

table at a constant velocity with 3.50N of force.a) Determine the normal force supporting the

textbook.b) Calculate the force of friction between the book and

the bench. c) Calculate the coefficient of friction between the

book and the bench. d) Which coefficient of friction have you found:

static or kinetic? Explain.

Answers

Answers

Starter/Review

From your lab… What things affect the coefficient of friction

(and therefore affect the amount of friction)?

Mass? Surface Area? Surface Type?

What do you think about the speed/velocity of the object – does this affect friction?

Friction - Summary The strength of friction depends on…

Surface materials (type of surface) Rough surface means more friction (ie, larger coefficient of

friction) Magnitude of forces pressing surfaces together (mass)

More mass means more friction The strength of friction DOES NOT depend on…

Surface area Velocity of object moving

See page 140, table 4.5 for a list!

Just some stuff to think about… Can I remember the following…

Explain Newton’s First law and give an example of this law.

What is the net force on Mrs. Evans when she is sitting down? Why?

What type of reference is this (inertial or non-inertial)?

Predict the motion (direction, is it slowing down or speeding up) that each object would undergo based on the free-body diagram provided:

a) b)

Quiz Tomorrow Friction (and Newton’s First Law since it

deals with friction) Friction – formula/questions FBD (to determine normal force) Coefficient of friction How does Newton’s First law apply to friction? What things affect friction?

Questions???