1. Friction

2. 1. A classroom desk supported by long legs is stationary in the room
1. A classroom desk supported by long legs is stationary in the room. A teacher comes around and pushes upon the desk in an effort to start it into a state of motion. The desk does not budge. The desk remains at rest because ______. a. there is a force of static friction opposing its motion b. there is a force of kinetic or sliding friction opposing its motion c. there is a force of rolling friction opposing its motion
2. A classroom desk supported by long legs is stationary in the room. A teacher comes around and pushes upon the desk in an effort to start it into a state of motion. The desk is finally accelerated from rest and then moves at a constant speed of 0.5 m/s. The desk maintains this constant speed because ______. a. there is a force of static friction balancing the teacher's forward push b. there is a force of kinetic or sliding friction balancing the teacher's forward push c. there is a force of rolling friction balancing the teacher's forward push d. the teacher must have stopped pushing
3. The symbol μ stands for the _____ a. coefficient of friction b. force of friction c. normal force
4. The units on μ are _____ a. Newton b. kg c. m/s/s d. ... nonsense! There are no units on μ.

3. Homework
N2L Video will be posted today - due at midnight

7. Static and Kinetic Friction Graph

8. Coefficient of Friction (p1)
1. What is the coefficient of friction for a steel block sliding along a steel floor? ___________
2. What is the coefficient of friction for a rubber tire at rest on a dry concrete surface? __________

10. Example
A 400 kg copper block is sliding on a level, steel floor. Calculate the magnitude of the horizontal force required to continue to move the crate across the floor at a constant speed of 2 meters per second.
Answer: 1411 N

11. Regents Practice Problems
Answer Key on the Door

12. A student and the waxed skis she is wearing have a combined weight of 850 newtons. The skier travels down a snow-covered hill and then glides at a constant velocity to the east across a snow-covered, horizontal surface.
1a. Determine the magnitude of the normal force exerted by the snow on the skis as the skier glides across the horizontal surface.
1b. Calculate the magnitude of the force of friction acting on the skis as the skier glides across the snow-covered, horizontal surface.
2. What is the magnitude of the force needed to keep a 60-newton rubber block moving across level, dry asphalt in a straight line at a constant speed of 2 meters per second?
3. A 10-kg rubber block is pulled horizontally at constant velocity across a sheet of ice. Calculate the magnitude of the force of friction acting on the block.
4. The coefficient of kinetic friction between a 780-newton crate and a level warehouse floor is Calculate the magnitude of the horizontal force required to move the crate across the floor at constant speed.
5. A 0.50-kilogram puck sliding on a horizontal shuffleboard court is slowed to rest by a frictional force of 1.2 newtons. What is the coefficient of kinetic friction between the puck and the surface of the shuffleboard court?
6. A skier on waxed skis is pulled at constant speed across level snow by a horizontal force of 39 newtons. Calculate the normal force exerted on the skier.
7. A N car with rubber tires is resting in a garage with a dry, concrete floor. What is the maximum force that can be applied to the car without it sliding across the floor?