GEN PHYSICS A STUDIO
GEN PHYSICS A STUDIO PHY 2048C
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This 10 page Class Notes was uploaded by Garett Kovacek on Thursday September 17, 2015. The Class Notes belongs to PHY 2048C at Florida State University taught by Simon Capstick in Fall. Since its upload, it has received 22 views. For similar materials see /class/205523/phy-2048c-florida-state-university in Physics 2 at Florida State University.
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Date Created: 09/17/15
Chapter 5 Applications of NeWton S Laws riction Prof Chris Wiebe Prof Simon Capstick v401ltmh 39 Ea fa 6 Circular motion Practice PRS Question review CONCEPTUAL CHECKPOINT The scale at left heads 981 N Is 9hr reading of the scale at righl a 3th than 981 N b equal to 931 N or c lastlian931 N Hint Think about AGreater B Equal C Less than 98 N Practice PRS Question review CONCEPTUAL CHECKPOINT The scale at left reads 981 N is the reading of the scale at ght a greater than 981 N in equal to 981 N or it less than 9 81 N Reasoning and Discussion Since a pulley simply changes the direction of the tension in a string without changing its nugnitu dc it is clear Jul the scale attached to the ceiling leads the same as the scale shown in the gure below There is no difference however between attaching the top end of the scale to somell39ling rigid and attaching it to another mookg hanging mass In either case the fact that the scale is at rest means that a force at 981 N must be exerted to the left on the topof the scale to balance the 981N folco cxcrloci on the lower and of Ihc scale Asa result he 1ch scales read the same Answer b The reading of luc scale al rlghl isaqual to 981 N Steps on a silicon surface Origin of friction Steps on Sil001 B S Swarlzentruher 0 Remember friction comes from electrostatic interactions between surfaces in contact with each other 0 Surfaces are rarely smooth at the atomic level jagged edges exist that interact with other surfaces Friction Static friction l ie pushing on a fridge that doesn t move l Friction is a oorly understood orcethat exists between ObJECtS In contact with each other It 0 poses motion due to sur ace roughness Two kinds 1 Static friction 2 Kinetic friction Static friction occurs before an object starts to move The static frictional force will match the applied force up to a threshold After this threshold it will start to move but the friction is called kinetic friction moving surfaces These two forces are different gt fs fS fapplied until a maXImum value IS reached This fIsmax s In f fsmax 5111 fsmax l fk kPn fa Fapp F app After it starts moving fk UK Fn Friction o In general the maximum size of static friction is greater than kinetic 0 Friction demo 0 Friction is proportional to the normal force on an object o For static friction fS S us Fn Coefficient of static frictionv a number typically 5 1 o For kinetic friction fK uK Fn Coefficient of kinetic friction again a number typically 5 1 An example of friction Y 0 Problem 525 ZF Fn Fg 0 F9 100 kg98 ms2 980 N n H ZFx Iapplied Ifriction x Which friction do we use F FS 5 p5 Fn 06 980 N friction 580 N F Fapplied 500 N 9 Therefore since Fapplied lt F5 FS matches Fapplied and is 500 N What happens if Fapplied gt 580 N The box starts to move kinetic friction PRS question 0 We usually think about friction as opposing motion Can friction ever cause an object to accelerate o A Yes 0 B No 0 Answer A Yes Think about for example a box sitting in the back of a moving truck that is accelerating Friction is the only force keeping the box in the truck so it causes the box to accelerate Friction demo ll 0 Can we calculate the coefficient of friction for a mass on an incline 0 There should be an angle 6 where an object will start sliding 0 Strategy 0 1 Isolate the body 0 2 Draw a free body diagram Friction demo 3 Write down the X and y components of the forces 4 Solve The maximum angle will be when the object starts to slide for the max f5 This is sometimes called the angle of reposequot mgsin G x mg 1ZFXmgsine fslmaxmax0 2ZFyFn mgcosemay0 Fn mgcose Sub into equation 1 for Fn mgsine fslmaxus Fn us mgcose Therefore us sinec056 tane max
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