4.8 Friction and inclines

FRICTION: The force that resists motion between two things in contact w/each other Friction is parallel to the surface Friction points opposite to the motion

COEFFICIENT OF FRICTION (μ) pronounced (mu) the ratio of the frictional resistance force to the normal force which presses the surfaces together There are no units b/c force divided by force. Usually a number between 0 and 1 Every different combination of surfaces has their own coefficient of sliding and static friction Depends on type of material, texture, and amount of lubricant

Kinetic Friction the friction that arises when two surfaces are sliding against each other Ff = μkFN FN = the force perpendicular force of the contact surface pushing against the object

Static Friction The force that opposes a force applied from rest (net force = 0) Ff = μsFN It’s like the force that you need to push with to get something to move, or the force that holds an object from sliding down an incline.

Frictionless Inclined Planes with Pulleys Assume for this problem that —that is, mass M will pull mass m up the slope.

Σ F = ma (no friction) Mg –T = Ma T-mgsin θ = ma If had been moving in the other direction: T-Mg =Ma mgsin θ – T = ma The 2 masses move with one acceleration. Substitute T in and solve for what is asked for. M = large mass (or m 1) m = small mass (or m 2) T = tension in string a = acceleration of system g = acceleration due to gravity θ = angle of incline

For inclines with friction find the acceleration of the masses: Determine the net force acting on each of the masses and apply Newton’s Second Law : Mass M: Mg –T = ma Mass m : T- mgsin θ –μN = ma T- mgsin θ –μmgcos θ = ma