Physics 121 Thursday February 7 2008 Carry on Luggage Frank L H Wolfs Department of Physics and Astronomy University of Rochester Physics 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 Velocity Frank L H Wolfs Department of Physics and Astronomy University of Rochester Physics 121 Course announcements Homework set 2 due on Saturday morning February 9 at 8 30 am Homework set 3 will be available on the WEB on Saturday morning February 9 at 8 30 am and will be due next week on Saturday morning February 16 at 8 30 am As part of homework set 3 you will be asked to download and install loggerPro from the Physics 121 website and install it on your computer The software runs on both Windows and Mac OSX We will be using this tool to start carrying out video analysis on homework set 4 Frank L H Wolfs Department of Physics and Astronomy University of Rochester Physics 121 Course announcements In order to access the restricted areas of the Physics 121 website containing solutions to homework assignments and exams you will need the use the username and password combinations distributed via email on Wednesday This same combination will give you access to the installers of the loggerPro software Frank L H Wolfs Department of Physics and Astronomy University of Rochester WeBWork set 3 All about friction Similar to problem 4 on set 2 but now with friction The normal force in this problem is directed horizontally Determine the net acceleration of the blocks in order to determine their contact force Frank L H Wolfs Motion with variable acceleration Make sure you determine the normal force correctly Make sure you deterMine the correct Directions of the friction and normal forces Department of Physics and Astronomy University of Rochester Physics 121 Quiz Lecture 5 The quiz today will have 3 questions Frank L H Wolfs Department of Physics and Astronomy University of Rochester A quick review Newton s first law of motion First Law Consider a body on which no net force acts If the body is at rest it will remain at rest If the body is moving with constant velocity it will continue to do so Notes Net force sum of ALL forces acting on the body An object at rest and an object moving with constant velocity both have no acceleration Frank L H Wolfs Department of Physics and Astronomy University of Rochester A quick review Newton s second law of motion Second Law The acceleration of an object is directly proportional to the net force acting on it and it inversely proportional to its mass The direction of the acceleration is in the direction of the net force acting on the object F m a Frank L H Wolfs F 3N a 5 m s2 F 4N m 1 kg Department of Physics and Astronomy University of Rochester A quick review Newton s third law of motion Third law Suppose a body A exerts a force FBA on body B Experiments show that in that case body B exerts a force FAB on body A These two forces are equal in magnitude and oppositely directed FBA FAB Note these forces act on different objects and they do not cancel each other Frank L H Wolfs Department of Physics and Astronomy University of Rochester Newton s laws of motion Problem solving strategies The first step in solving problems involving forces is to determine all the forces that act on the object s involved The forces acting on the object s of interest are drawn into a free body diagram Apply Newton s second law to the sum of to forces acting on each object of interest Frank L H Wolfs Department of Physics and Astronomy University of Rochester Newton s laws of motion Interesting effects The rope must always sag Why Frank L H Wolfs Department of Physics and Astronomy University of Rochester Newton s laws of motion Interesting effects The force you need to supply increases when the height of your backpack Increases Why Frank L H Wolfs Department of Physics and Astronomy University of Rochester Friction A block on a table may not start to move when we apply a small force on it This means that there is no net force in the horizontal direction and that the applied force is balanced by another force This other force must change its magnitude and direction based on the direction and magnitude applied force If the applied force is large enough the block will start to move and accelerate Frank L H Wolfs Department of Physics and Astronomy University of Rochester Friction When the applied force exceeds a certain maximum value the object will start to move Once the object starts to move the magnitude of the force required to keep the object moving with constant velocity is smaller than the magnitude of the force required to start the motion The forces that try to oppose our motion are the friction forces between the object and surface on which it is resting Frank L H Wolfs Department of Physics and Astronomy University of Rochester Friction Based on these observations we can conclude There are two different friction forces the static friction force no motion and the kinetic friction force motion The static friction force increases with the applied force but has a maximum value The kinetic friction force is independent of the applied force and has a magnitude that is less than the maximum static friction force Frank L H Wolfs Department of Physics and Astronomy University of Rochester Friction and braking Consider how you stop in your car The contact force between the tires and the road is the static friction force for most normal drivers It is this force that provides the acceleration required to reduce the speed of your car The maximum static friction force is larger than the kinetic friction force As a result your are much more effective stopping your car when you can use static friction instead of kinetic friction e g when your wheels lock up Frank L H Wolfs Department of Physics and Astronomy University of Rochester Friction and normal forces The maximum static friction force and the kinetic friction force are proportional to the normal force Changes in the normal force will thus result in changes in the friction forces NOTE The normal force will be always perpendicular to the surface The friction force will be always opposite to the direction of potential motion Frank L H Wolfs Department of Physics and Astronomy University of Rochester Pushing or pulling a big difference More Friction Frank L H Wolfs