1. Su-Jin Kim Gyeongsang National University Fall 2017
6. Friction
Su-Jin Kim
Gyeongsang National University
Fall 2017

2. OCW
Dry Friction
Inclined surface, Two boxes

3. MIT
Contact force
Static friction

4. Dry Friction
Friction force = Coefficient of friction x Normal force : F = μ N
Ex) F = 0.2 x 600 N = 120 N W F μ N

5. Increase Friction
Break
Clutch
P = n F
n = 10

6. Decrease Friction
Lubricant (Oil) μ↘
Wheel , Bearing ↘ P F

7. Tip over Slide or Tip over ? ΣM = W x – P h < 0  Tip over P W h FN

8. Static vs. Kinetic friction
Static μs > Kinetic μk Friction coefficient
F s = μs N
F k = μk N F
stop
sliding

9. Ex1) Inclined surface μ = F / N = tan θ = 6/30 = 0.2 (0.4)
Static > Kinetic : μs > μk W
6 (12) θ N θ W F 30

10. Ex2) Two boxes Top box slide only ? Top box tips over ?
Both boxes slide over ?
Both boxes tip over together ? P

11. Ex3) Ladder slide
The ladder can slide when a person climbs on it. Explain the relationship between the coefficient of friction and the installation angle of the ladder.

12. Mop
Calculate the force when pushing and pulling along the angle θ. The weight of the mop is W, the coefficient of friction is μ, and the weight of the rod is ignored. F W θ