2.  Friction – force that opposes motion  Caused by microscopic irregularities of a surface  The friction force is the force exerted by a surface as an object moves across it or makes an effort to move across it. Friction

3. 

4.   Depends on the normal force and the type of surface Ff=FNFf=FN  F f – force of friction (N)  F N – normal force   – coefficient of friction (1 or less) Friction

5.  Coefficient of Friction

6.   Three types of friction 1.Static – object at rest 2.Kinetic – object in motion 3.Rolling- object on wheels Types of Friction

7.   Friction depends only on  1) the nature of the two surfaces in contact  2) the normal force  It does not depend on the surface area. Friction

8.   If the coefficient of kinetic friction between a 35-kg crate and the floor is 0.30, what horizontal force is required to move the crate to the right at a constant speed across the floor?  If the object is on a level  surface, then the normal  force = weight force. If the  object is moving at constant  speed, then frictional force =  only force.  F f =  F N F N = mg  F f = (0.30)(35)(9.8) F N = (35) (9.8)  F f = 102.9 N Friction and Newton’s First Law

9.   Suppose the same 35 kg crate was not moving at constant speed but accelerating at 0.70 m/s 2. Calculate the applied force.  For horizontal forces with friction: F app = F net + F f  F app = ma +  F N  F app = ma +  mg  F app = (35) (0.70) + (0.3)(35)(9.8)  F app = 24.5 + 102.9 = 127.4 N Friction and Newton’s Second Law