Friction and Inclined Planes Friction force that opposes

Friction and Inclined Planes

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

Friction Depends on the normal force and the type of surface Ff=m. FN Ff – force of friction (N) FN – normal force m – coefficient of friction (1 or less)

Coefficient of Friction

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

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 and Newton’s First Law 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 = m. FN FN = mg Ff = (0. 30)(35)(9. 8) FN= (35) (9. 8) F f = 102. 9 N

Friction and Newton’s Second Law 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: Fapp= Fnet + Ff Fapp = ma + m. FN Fapp = ma + mmg Fapp= (35) (0. 70) + (0. 3)(35)(9. 8) Fapp= 24. 5 + 102. 9 = 127. 4 N