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ROCHESTER PHY 121 - PHY 121 Lecture 6 Notes

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1Frank L. H. Wolfs Department of Physics and Astronomy, University of RochesterPhysics 121. Thursday, February 7, 2008.Carry-onLuggage.Frank L. H. Wolfs Department of Physics and Astronomy, University of RochesterPhysics 121.Thursday, February 7, 2008.• Topics:• Course announcements• Quiz• Newton’s Law of Motion:• Review of Newton’s First, Second, and Third Law of Motion• Problem Solving Strategies• Friction:• Static and Kinetic Friction• Rounding a Curve• Terminal VelocityFrank L. H. Wolfs Department of Physics and Astronomy, University of RochesterPhysics 121.Course announcements.• Homework set # 2 due on Saturday morning, February 9, at8.30 am.• Homework set # 3 will be available on the WEB onSaturday morning, February 9, at 8.30 am and will be duenext week on Saturday morning, February 16, at 8.30 am.• As part of homework set # 3 you will be asked to downloadand install loggerPro from the Physics 121 website andinstall it on your computer. The software runs on bothWindows and Mac OSX. We will be using this tool to startcarrying out video analysis on homework set # 4.2Frank L. H. Wolfs Department of Physics and Astronomy, University of RochesterPhysics 121.Course announcements.• In order to access the restricted areas of the Physics 121website (containing solutions to homework assignments andexams) you will need the use the username and passwordcombinations distributed via email on Wednesday.• This same combination will give you access to the installersof the loggerPro software.Frank L. H. Wolfs Department of Physics and Astronomy, University of RochesterWeBWork set # 3.All about friction.Similar to problem4 on set # 2, but nowwith friction.Make sure you determine the normalforce correctly!The normal force inthis problem is directed horizontally.Determine the netacceleration of theblocks in order todetermine their contact force.Motion with variableacceleration!Make sure you deter-Mine the correct Directions of the friction and normal forces.Frank L. H. Wolfs Department of Physics and Astronomy, University of RochesterPhysics 121.Quiz Lecture 5.• The quiz today will have 3 questions.3Frank L. H. Wolfs Department of Physics and Astronomy, University of RochesterA quick review:Newton’s first law of motion.First Law:Consider a body on which no netforce acts. If the body is at rest,it will remain at rest. If the bodyis moving with constant velocity,it will continue to do so.• Notes:• Net force: sum of ALL forcesacting on the body.• An object at rest and an objectmoving with constant velocityboth have no acceleration.Frank L. H. Wolfs Department of Physics and Astronomy, University of RochesterA quick review:Newton’s second law of motion.Second Law:The acceleration of an object isdirectly proportional to the netforce acting on it and it inverselyproportional to its mass. Thedirection of the acceleration is inthe direction of the net forceacting on the object:F = 4 NF = 3 Na = 5 m/s2m = 1 kg !F!= m!aFrank L. H. Wolfs Department of Physics and Astronomy, University of RochesterA quick review:Newton’s third law of motion.Third law:Suppose a body A exerts a force(FBA) on body B. Experimentsshow that in that case body Bexerts a force (FAB) on body A.These two forces are equal inmagnitude and oppositelydirected:Note: these forces act ondifferent objects and they do notcancel each other. !FBA= !!FAB4Frank L. H. Wolfs Department of Physics and Astronomy, University of RochesterNewton’s laws of motion.Problem solving strategies.• The first step in solving problems involving forces is to determine all theforces that act on the object(s) involved.• The forces acting on the object(s) of interest are drawn into a free-bodydiagram.• Apply Newton’s second law to the sum of to forces acting on eachobject of interest.Frank L. H. Wolfs Department of Physics and Astronomy, University of RochesterNewton’s laws of motion.Interesting effects.The rope must always sag!Why?Frank L. H. Wolfs Department of Physics and Astronomy, University of RochesterNewton’s laws of motion.Interesting effects.The force you need to supply increaseswhen the height ofyour backpack Increases. Why?5Frank L. H. Wolfs Department of Physics and Astronomy, University of RochesterFriction.• A block on a table may not startto move when we apply a smallforce on it.• This means that there is no netforce in the horizontal direction,and that the applied force isbalanced by another force.• This other force must change itsmagnitude and direction based onthe direction and magnitudeapplied force.• If the applied force is largeenough, the block will start tomove and accelerate.Frank L. H. Wolfs Department of Physics and Astronomy, University of RochesterFriction.• When the applied force exceeds acertain maximum value, theobject will start to move.• Once the object starts to move,the magnitude of the forcerequired to keep the objectmoving with constant velocity issmaller than the magnitude of theforce required to start the motion.• The forces that try to oppose ourmotion are the friction forcesbetween the object and surface onwhich it is resting.Frank L. H. Wolfs Department of Physics and Astronomy, University of RochesterFriction.• Based on these observations wecan conclude :• There are two different frictionforces: the static friction force(no motion) and the kineticfriction force (motion).• The static friction force increaseswith the applied force but has amaximum value.• The kinetic friction force isindependent of the applied force,and has a magnitude that is lessthan the maximum static frictionforce.6Frank L. H. Wolfs Department of Physics and Astronomy, University of RochesterFriction and braking.• Consider how you stop in yourcar:• The contact force between thetires and the road is the staticfriction force (for most normaldrivers). It is this force thatprovides the acceleration requiredto reduce the speed of your car.• The maximum static friction forceis larger than the kinetic frictionforce. As a result, your are muchmore effective stopping your carwhen you can use static frictioninstead of kinetic friction (e.g.when your wheels lock up).Frank L.


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