If you push lightly on an object does it move?
When you push lightly on the object, are the forces balanced?
Several factors can cause friction.
What do you think causes friction?
What are some high friction surfaces? Some low friction surfaces?
What do you think causes friction now?
Demonstrations – What causes the retarding / opposing force in each situation?
Rough Surfaces – Think of sliding on grass vs gravel
The three causes of friction:
Interactions of “bumps”
Creation and Destruction of welds
Intermolecular Forces
Friction is a force that opposes motion.
There are 2 different types of friction, Static and Kinetic
Static Friction: the force that resists the initiation of sliding motion between two surfaces that are in contact and at rest.
Kinetic Friction: the force that opposes the movement of two surfaces that are in contact and are sliding over each other.
The force of friction is proportional to the normal force
Is it easier to push a chair across the floor empty or with someone in it?
The force of friction can be approximated and is dependent on the surfaces involved. o The force of friction depends on the composition of the surfaces intact. o EX: it is easier to push a desk across a tile floor then carpet o The coefficient of friction is the quantity that expresses the dependence of frictional forces on the particular surfaces in contact
Coefficient of friction (
μ)
the ratio of the magnitude of the force of friction between two objects in contact to the magnitude of the normal force with which the objects press against each other.

The coefficient of friction is a ratio of forces
The coefficient of kinetic friction is a ratio of the force of kinetic friction and the normal force
k
k
N kinetic coefficient = Force of kinetic friction / normal force
The coefficient of static friction is a ratio of the force of static friction and the normal force
s
s(max)
N static coefficient = Max Force of static friction / normal force
Table 4.2 on page 104 gives coefficients of friction that you will need for homework problems
Example: a) A 24 kg crate initially at rest on a horizontal floor requires a 75 N horizontal force to set it in motion. Find the coefficient of static friction between the crate and the floor
0.31 b) Once the crate is moving it only requires 63 N of horizontal force to keep it moving at a constant velocity. What is the kinetic coefficient of friction between the crate and the floor?
0.27 c) The same crate is now pulled with a rope that is 30 ° above the horizontal. If the coefficients of friction are the same, then a) what must the tension in the rope be to get the crate to start moving? b) what must the tension in the rope be to keep the crate moving? a) 71.41 N b) 63.33 N
Homework: page 128 35-38, 40-43
Bring in Mousetrap cars for testing on Monday!! You may take the cars home
Monday night, but NOT Tuesday night. Make sure you have read all of the rules and that you have a Victor Brand Mousetrap for your car. Any other brand of mousetrap will result in a non-droppable zero!!