1. SPH3U – Unit 2
Friction

2. Friction
Friction the force that opposes motion whenever one surface moves against another.

3. Friction Sliding Friction Rolling Friction Fluid Friction
Rolling Friction
Fluid Friction
Makes it difficult to slide one surface past another.
Makes it difficult to roll one surface past another.
Makes it difficult to move an object in a fluid.

4. Friction
Static Friction  The force that prevents an object from starting to move.
Limiting Static Friction  The maximum value of the force of friction just before an object starts to move.

5. Friction
Kinetic Friction  the force of friction that opposes the movement of objects once they are in motion. It is always in the opposite direction to the motion.
Normal Force  force at 90o angle [up] from the surface an object is on.

6. Friction
For any two surfaces, the ratio of the size of the force of kinetic friction to the size of the normal force is a constant. This is because the force of friction is directly proportional to the normal force. If the force of gravity doubles, then the normal force doubles, and the force of kinetic friction also doubles. This fact gives us the two constant ratios: coefficient of kinetic friction, and coefficient of static friction.

7. Friction
Coefficient of Kinetic Friction (μk)  This equation relates the magnitudes (not directions) of the Force of Friction, Ff, the normal force, Fn, and the coefficient of kinetic friction.
The coefficients of friction are different for different substances.
See pg. 170

8. Friction
Coefficient of Static Friction (μs)  This equation relates the magnitudes of the
Maximum Force of Friction, Ff, just before the object moves, the normal force,
Fn, and the coefficient of static friction.

9. Example Problem
A car moves down the street and begins to break along the asphalt. The car has a mass of 3 x 103 kg. Find the force of kinetic friction on the car.

10. Example Problem
A car moves down the street and begins to break along the asphalt. The car has a mass of 3 x 103 kg. Find the force of kinetic friction on the car.

11. Homework
Complete Questions:
Pg. 172 # 1,2-8