1. Where the Rubber Meets the Road
Friction

2. Friction A force that OPPOSES MOTION. What causes friction?
Attraction between protons and electrons in the object and surface

3. Friction What causes friction?
2. Small ridges and bumps at the microscopic level cause objects and surfaces to catch on one another.

4. Friction Friction force strength
How “sticky” are the surfaces involved?
Coefficient of friction (μ) – NO UNITS
How hard are the surfaces pushing together?
NORMAL FORCE
Are the surfaces sliding against each other?
KINETIC vs. STATIC

5. Which Coefficient? Kinetic Static Used ONLY for SLIDING objects
Pulled along a surface
Skidding
Static
Used for any NON-SLIDING objects
Not moving
Rolling

6. Example #1
A 20 newton wooden box is sliding across a wooden surface. What is the friction force acting on the box?
Ff = μFN
Ff = (0.3)(20 N)
Ff = 6.0 N

7. Example #2
A 40 newton copper box is placed on a flat, steel table. What is the minimum force needed to make the box move?
Ff = μFN
Ff = (0.53)(40 N)
Ff = 21 N

8. End of PRACTICE