1. Friction

2. I. Friction A. Background 1. A force that acts opposite the direction of movement 2. Friction slows you down and causes heat

3. 3. The normal force is the force between 2 objects in contact with each other 4. On a horizontal surface, the normal force acts equal and opposite to the force of gravity (weight, Fg) Fn Fg

4. B. Coefficient of Friction 1. The force due to friction, Ff, and the normal force, Fn, have a direct relationship 2. Coefficient of friction, μ (mu), has no units 3. Formula: F f = μF n

5. C. Static and Kinetic Friction 1. Static friction  Force that opposes the start of motion 2. Kinetic Friction  The friction between objects in contact when they are moving

6. 3. Kinetic friction decreases as motion begins 4. Coefficient of friction is greater for static friction than kinetic friction

7. Examples Wood 10kg Wood What is the force of friction needed to make the object move from rest? What is the force of friction needed to keep the object moving?

8.  The diagram shows a 4kg object accelerating at 10 meters per second squared on a rough horizontal surface. What is the frictional force acting on the object? a = 10 m/s 2 Frictional Force, FfApplied force, 50N M = 4kg

9.  4. A sled sits on packed snow. The coefficient of kinetic friction is 0.12. If a person and the sled together weigh 650N, what force is needed to pull the sled across the snow at constant speed?

10.  5. A 1.4kg block slides across a rough surface such that is slows down with an acceleration of 1.25m/s2. What is the coefficient of kinetic friction between the block and the surface?

11. D. Friction on an Incline 1. On an incline, weight still acts straight downward, towards the surface of the earth.

12.  8. Stacie, who has a mass of 45kg, starts down a slide that has an angle of 45º with the horizontal. If the coefficient of kinetic friction between Stacie’s shorts and the slide is 0.25, what is her acceleration?