APPLICATION OF FRICTIONLaws of FrictionProblems involving Static and Kinetic Friction

LAWS OF FRICTION:1. Friction is parallel to the surfaces that are in contact

and is in the opposite direction of the motion of the object.

2. Friction depends on the nature of the materials in contact.

3. The sliding friction is usually less than the starting friction.

4. Sliding friction is independent of speed.5. Friction is practically independent of the area of

contact.6. Friction is directly proportional to the force pressing

surfaces together.

COEFFICIENT OF FRICTION

DEFINITION OF TERMSCoefficient of friction- ratio of frictional force and the normal force

Normal force- perpendicular force pressing the surfaces together

FORMULAFor static friction:

For kinetic friction:

SAMPLE PROBLEM #1As Nestor is taking a bath, the soap falls out of the soap dish and he steps on it with a force of 400 N. If Nestor slides forward and the frictional force between the soap and the floor in 40N, what is the coefficient of the friction between these two surfaces.

Given: Fn= 400 N Ff= 40 NSolution:

SAMPLE PROBLEM #2At an animal circus show, a 900kg sea lion slides down a wet slide inclined at an angle of 28° to the horizontal. The coefficient of friction between the sea lion and the slide is 0.40. What frictional force slows the sea lion’s motion down the slide?

Given: m= 900kg = 28° g= 9.8m/s2 = 0.40Solution: Fn= (900)(9.8)cos 28° Fn= 7787.60NFf= Fn Ff= (0.40)(7787.60)Ff= 3115.04 N

PRACTICE EXERCISE1. A man prevents a 15kg wood from falling by

pressing it against a vertical wall. What force must he use if the coefficient friction is 0.3?

2. The coefficient sliding friction between a loaded refrigerator and the floor is 0.40. If a normal force of 60 N acting 35° above the horizontal gives the refrigerator a constant velocity, what is the weight of the loaded refrigerator?