tutorial3_sp10 - Physics 2010 – Spring 2010 Recitation...

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Unformatted text preview: Physics 2010 – Spring 2010 Recitation Assignment 3 NAME ________________________ Section Day (circle): M Tu W Th F Section Time: 8a 10a 12p 2p 4p TA Name:_____________________ Work together in groups of 3 or 4! Use the space provided for all of your answers. This week's assignment covers forces, Newton's laws, and the use of free-body diagrams. (Ch. 4) Remember that free-body diagrams should show only one object and the (labeled) vector forces on that object! 1. Drawing free-body diagrams for objects at rest. a) Draw a free-body diagram for a cart of mass m resting on a table. (The free-body diagram is of the cart.) Label every force with the type of force (normal, gravitational, etc.). b) How does your free-body diagram show the relative magnitude of the forces? What observation did you make to get information about the relative magnitude of the forces? Express this information as an equation. c) A brick of mass M (heavier than the cart) is placed on top of the cart. The table does not collapse. Now draw two free-body diagrams: one for the cart, the other for the brick. Label each force clearly with its cause and effect. For example, the force of the table on the cart should be labeled as “Ftable on cart”. d) Explain why no force that appears on one diagram should appear on the other. 1 e) What type of force (e.g. frictional, gravitational, etc.) does the brick exert on the cart? (Hint: it's not gravitational. Why not?) f) Does any force on the cart have the same magnitude as a force on the brick? Which? g) Using the result for f), write an expression for the normal force the table exerts on the cart in terms of m, M and g. h) Now think of the cart and brick together as a single object, whose mass is now m+M. How does the normal force exerted by the table on this single object compare to your answer for part g)? i) Now, suppose the table collapsed when the brick was placed on the cart. How could you explain that in terms of the forces on your free-body diagram? 2. Drawing free-body diagrams for objects in motion. The following questions refer to a system of three identical bricks, stacked as shown in Fig. 1, being pushed across a table horizontally at a constant speed. Each brick has mass m. Call the stack of two bricks System A, and the single brick on the right System B. There is a frictional force with coefficient µ. a) Compare the net force on System A to the net force on System B. How did you come to this conclusion? Fig. 1. Three bricks stacked, being pushed by the hand at constant speed. b) Draw separate free-body diagrams for System A and System B. Label each force by identifying the type of force and the object exerting the force (for example, Ffriction on A). 2 c) Is the magnitude FAB , the force exerted on System B by System A greater than, less than, or equal to the magnitude FBA , the force exerted on System A by System B? Why? d) The forces FAB and FBA are often called action-reaction forces or force pairs. Newton’s Third Law is referring to these force pairs when stating that “every action has an equal and opposite reaction”. Identify all the action-reaction force pairs in the problem and list them in the table below. Every force on your free-body diagrams will be listed on the table, but not all the “reaction” forces will be in the diagrams you drew. List these pairs anyway. One example is done for you. “Action” Force (One of the “Force Pair” forces from FBD) Force of table on system A “Reaction” Force (The corresponding “Force Pair” force) Force of system A on table e) Why are the words “ACTION” and “REACTION” written in quotes in the table above? f) What criteria did you use to identify the force pairs? Is your result in part c) consistent with your identification of force pairs? g) Now, treat all three bricks as one System C. Draw a free-body diagram, and compare the individual forces and net force on C to your answers in part b). List which forces in part b) correspond to each force on System C. 3 h) Are there any forces that were present on your diagrams in part b), but not in part g)? What characteristic do these forces share, and why are they unimportant to the motion of System C? i) For the remaining problems, assume that the system is set up as in Fig. 1, except that all the frictional forces become smaller. In particular, note that the force applied by the hand on System A is not changed. Draw the free-body diagrams for Systems A and B. j) The motion of the system has now changed. Describe the motion in a couple of words. k) Is the net force on System A bigger, smaller or the same as the net force on system B? How can you tell? l) Identify and describe the flaw in this statement: "System A and System B are pushed by the same force as before, so the motion will be the same as before." m) Identify and describe the flaw in this statement: "System A and System B should be speeding up since friction is lower. So now System A is pushing on System B with a greater force than System B is pushing on System A." 4 ...
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This note was uploaded on 08/24/2010 for the course PHYS 2010 taught by Professor Dubson during the Spring '06 term at Colorado.

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