Unit Two: DynamicsPart 3: Friction

Friction – Quick Review A contact force

Electromagnetic Force (between surface atoms of objects touching)

Always opposes motion

Friction There are 2 types of friction:

Static Frictional Force When you start to move an object from rest Larger than Kinetic Frictional Force due to Inertia ųs

Kinetic Frictional Force Exists when the object is moving 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 “Stickiness value” ų (symbol mu) ų has no units

Page 140, table 4.5

Formula: Ff = ųFN

Remember: 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 52 kg. 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!!!

Starter/Review

Starter/Review

Starter/Review

Friction The strength of friction depends on…

Surface materials Magnitude of forces pressing surfaces together

The strength of friction DOES NOT depend on… Surface area Velocity of object moving

See page 140, table 4.5 for a list!