1. Poll You stand vertically on a scale on a hill of angle . The normal component of the force of the hill on you has magnitude 1.mgsin(  ) 2.mgcos(  ) 3.mg 4.zero

2. Poll You stand vertically on a scale on a hill of angle . The frictional component of the force of the hill on you has magnitude 1.mgsin(  ) 2.mgcos(  ) 3.mg 4.zero

3. Friction FfFf F N, on book by table The tangential (i.e. parallel) component of the force on the book by the table is friction. Friction depends on: (1)the surfaces in contact (2)the normal component of the contact force

4. Coefficient of friction If the surfaces are sliding past each other If surfaces are not sliding past each other but are just about to slide If surfaces are not sliding and are not about to slide

5. Poll A four-wheeler (i.e. ATV) is parked at rest on a slightly inclined hill. If the hill were much steeper, the ATV would still be parked at rest, without sliding. The frictional force on the ATV in this case is 1.f k =  k F N 2.f s =  s F N 3.f s <  s F N 4.f k <  k F N

6. Poll A four-wheeler (i.e. ATV) is parked at rest on a steep hill. If the hill were any steeper, the ATV would slide down the hill. The frictional force on the ATV in this case is 1.f k =  k F N 2.f s =  s F N 3.f s <  s F N 4.f k <  k F N

7. Poll A four-wheeler (i.e. ATV) is sliding down a steep hill at a constant speed. The frictional force on the ATV in this case is 1.f k =  k F N 2.f s =  s F N 3.f s <  s F N 4.f k <  k F N

8. Poll A four-wheeler (i.e. ATV) is sliding down a steep hill and speeding up as it travels down the hill. The frictional force on the ATV in this case is 1.f k =  k F N 2.f s =  s F N 3.f s <  s F N 4.f k <  k F N

9. Poll A four-wheeler (i.e. ATV) is sliding down a steep hill and slowing down as it travels down the hill. The frictional force on the ATV in this case is 1.f k =  k F N 2.f s =  s F N 3.f s <  s F N 4.f k <  k F N

10. Example If the gymnast’s mass is 70 kg and if his arms make an angle of 35  with respect to the floor, what is the frictional force on his hands?

11. Example Suppose that the gymnast finds that if his arms make an angle of 25  with respect to the floor, his hands will slip. What must be the coefficient of static friction between his hands and the floor?

12. Example Suppose that a 80-kg man skis down a 20  hill with a constant speed. What is the coefficient of kinetic friction between his skis and the hill?

13. Example Suppose that a 80-kg man skis straight down a 30  hill with an acceleration 3 m/s 2. What is the coefficient of kinetic friction between his skis and the hill?