7Source of FrictionAt the microscopic level even the smoothest of surfaces is dotted with little “mountain peaks”.The tips of the peaks are the only parts that touch the other material.Only a very small portion of the apparent surface area is in contact with the other surfaceThis causes extremely high pressures to form on the parts that touch. This causes the two surfaces to become “welded” almost at the points of contactPicture of extremely smooth mica with a scanned probe microscope
8Source of friction (continued) The true surface contact area is proportional to the normal force because the peaks will deform plastically when force is applied increasing the contact areaPlastic deformation: to change shape permanently without fracturingOther lesser reasons for frictionSurface adhesion between pure metalsPloughing of one surface by the other harder oneElastic deformation
9Experiment: Does the friction force depend on surface area? We stick a 1kg mass on a piece of plexiglass on top of the clean surface and, using a Newton meter, pull the weight across the table with the string provided at constant speed.We put the same weight on a different piece of plexiglass with drastically different surface area and repeatWhat do you notice?
10Friction doesn’t depend on apparent surface area (much) You may have noticed a small dependence. This is partially due to the greater weight of the larger piece of plexiglass, and also partially due to the fact that there is a dependence of friction on surface area, just one that is small enough to be ignored under most circumstances.Why is the friction force approximately independent of surface area? Because as discussed previously the materials plastically deform at the peaks depending on the pressure increasing real surface area.RSA = real surface area, F = Normal Force, SA = Surface area, F/SA = P Pressure, C = constant depending on the materials
11What is k and s?The coefficient of friction is different depending on if the objects are in motion relative to each other or not as in the graph. s is the coefficient of static friction and k is the coefficient of kinetic friction.
12Why?The slightly larger value for static friction results from irregularities and contaminants on the surfaces and is less accurate in general than the coefficient of kinetic frictionBetween (very) carefully cleaned surfaces the difference between the two coefficients disappears
13Finally something useful.. The only formula you need for friction forces:orFf = Fn
15Newton’s Second and Third Laws Newton’s Second Law: If there is an unbalanced force on an object it will accelerate according to FUnbalanced =Newton’s Third Law: If you exert a force on an object it exerts an equal and opposite force on you
17Normal ForceFrom Newton’s third law we know that if gravity or some other force pushes an object (like a block) into a second object (like a table) that second object will be exerting an equal force back on the first.Normal force is the force the table exerts back on the blockNormal force is always exerted perpendicular to the surfaceFriction Force is always parallel to the surfaceSo if the table is horizontal and gravity is the only force on the block Fn = -Fg
18Normal force on a hillNormal force is exerted perpendicular to the surface in accordance with Newton’s Third lawNo unbalanced force so the block is stationary or at least not accelerating
19Typical Physics question Ancient Egyptians are pushing a stone block up the side of a pyramid of angle 20 degrees. What stone should they be using so they can take a break and not have it slide down, but have it still be not much over the coefficient of friction needed so they don’t have to push so hard?Stone AStone BStone CStone D
21ExperimentWith the Newton meter find the coefficient of kinetic and static friction for the plexiglass and the plastic track.
22Wormbot? Wormbot is a crawling machine divided into three sections. Each section weights 3.67 kgEach section has a plexiglass bottom layer so they each have the same coefficient of friction that we found earlier for plexiglass on the surface.
23Can Wormbot gain a net movement in a direction? Why is this possible/impossible?
24If you have two weights with the coefficient of friction connected by something that can expand and contract can you gain a net movement?What if you have 4 weights with connections in between?What if you have three weights but one is heavier than the other two. How much heavier can it be?
25How much weight can it pull on flat ground with the corner pulley How much weight can it pull on flat ground with the corner pulley? Does it matter where the additional weight is attached? How does putting the weight on plexiglass on the table and getting it to tow it that way change things?
26If you were to put Wormbot on a table at an angle what is the steepest angle he could climb?