1. Friction Animation: Friction at PhET. Discuss.
Demo: Static vs. kinetic friction.

2. Static Friction - Examples
1. You are a rock climber, climbing a 'chimney', which is a gap between two stone walls, with no hand holds or grips. How do you manage to stay at rest? 
You push the stone and the stone pushes back. This creates static friction that keeps you there.
2. Hold a book against a wall by applying a force perpendicular to the wall. How does the book stay at rest?
You push the book and the wall pushes back. This creates static friction that keeps it there.
3. A car makes a turn on a dry asphalt road. What force keeps the car on the road so it doesn’t slip?
Static friction between the tires and the road.

3. Static Friction - Examples
4. Are the static frictional forces of the same magnitude? 
The static frictional forces are not of the same magnitude. The more you pull on something the more static frictional force appears. When the movement is about to begin Ffs becomes maximum value. 

4. Kinetic Friction - Examples
1. Explain this cartoon. 
When the lawnmower is moving the frictional force is kinetic and it has a value Ffk<Ffsmax.
2. A sled and its runner slow down from 4.0 m/s to rest for 5. s. What makes the system come to a stop?
The kinetic frictional force that opposes the motion, therefore is directed to the left.

5. Kinetic Friction - Examples
3. A driver steps on the gas pedal and goes up to 60 mi/h in 8.0 s. What forces act on the car horizontally during this interval? 
4. A driver steps off the gas pedal and puts the car in neutral N. The car goes down to 0 mi/h in 20. s. What forces act on the car horizontally during this interval? 

6. Friction