Lab05-NewtonsLaws-LabExploration-InPerson

Lab 5: Interacting Bodies Physics 112 Lab Partner ’ s Names: Copyright 2005 by S.E. Kanim, M.E. Loverude, & L.G. Ortiz 1 1. Introduction In this laboratory, you will examine situations in which two bodies are interacting as they move in simple ways. Newton’s second and third laws are some of the most difficult and widely misunderstood topics in introductory physics. Many students are able to recall one or more statements of these laws, but applying the laws to physical situations proves more difficult than most students and instructors might expect. The intuition that many students develop watching objects in the real world often fails. For example, we all learn at a very early age that you must push on something to make it move. Many students generalize this experience and infer a direct relationship between force and velocity. However, Newton’s second law relates force to acceleration rather than to velocity. Similarly, Newton’s third law states that the forces of interaction between two bodies are equal. That’s easy enough to accept when the bodies are at rest, and have the same mass, but can be confusing in real world situations, for example when a heavy truck moving at 60 mph smashes into a small car parked by the curb. In order to understand this situation, it is important to be clear on the definitions and differences between normal force, net force, and acceleration. Equipment This lab combines a number of pieces of equipment and sensors that you have used before, including force probes, which connect to your computer wirelessly. Generally, you will need to: • Launch the capstone software • Connect the force probe wirelessly and make sure capstone “sees” it • Choose an appropriate display as described in each section below Consult your instructor if you have difficulties.

Lab: Interacting Bodies Physics 112 Copyright 2005 by S.E. Kanim, M.E. Loverude, & L.G. Ortiz 2 2. Interacting objects: constant speed In this section, you will perform an analysis of the forces acting when two bodies move together. You will be using two wooden sliding carts with force probes attached as shown below. The masses of the carts are written on the carts. The mass of a force probe is 200g (= 0.2 kg). One of the carts should include an additional 500-gram mass. We will call the more massive system (consisting of the wooden cart, force probe, and 500-g mass) on the left cart A and the less massive system (consisting of the wooden cart and force probe) on the right cart B . 2.1: Determine the total mass of each cart that you will be using for this experiment. Record these mass values in the boxes provided. Push the carts horizontally so that they move to the right at constant speed. Note that there is friction between the carts and the track. 2.2: Compare the net force (magnitude and direction) on system A to that on system B while they are moving at constant speed. Explain how you arrived at your comparison. 2.3: Draw separate free-body diagrams for system A and system B. Label each of the forces in your diagrams by identifying: the type of force, the object on which the force is exerted, and the object exerting the force. Include a key for your labels to the right of the diagrams if necessary. 2.4: Would you expect the magnitude of the force exerted on system A by system B to be greater than, less than, or equal to the magnitude of the force exerted on system B by system A? Explain.

Lab: Interacting Bodies Physics 112 Copyright 2005 by S.E. Kanim, M.E. Loverude, & L.G. Ortiz 4 3.3: You will now use Newton’s Laws to compare the magnitudes of all of the horizontal forces that you identified on your free-body diagrams in part 2.3. First, list all of the horizontal forces: Identify the Newton’s 3 rd Law pair of forces. Then, write the Newton ’ s 3 rd Law equation that relates the magnitudes of the forces you identified. (Hint: a 3 rd Law pair takes one force from each of your two free-body diagrams.) Write a Newton’s 2 nd Law equation comparing the magnitudes of the horizontal forces on cart B. ( Σ F cart B = m cart B * a cart B ) ( Hint: Recall that the carts move at constant speed.) Write a Newton’s 2 nd Law equation comparing the magnitudes of the horizontal forces on cart A. ( Σ F cart A = m cart A * a cart A ) ( Hint: Recall that the carts move at constant speed.) Of the horizontal forces on your free-body diagrams for carts A and B, which one is the largest? Explain.

Lab: Interacting Bodies Physics 112 Copyright 2005 by S.E. Kanim, M.E. Loverude, & L.G. Ortiz 5 3.4: Determine the magnitude of each of the forces, horizontal and vertical , that you drew on your free-body diagrams in part 2.3. For simplicity, use the approximation g = 10 m/s 2 . (Hint: you will need the information your measurements in 3.2, 3.3, as well as the formulas for weight and friction force.) Based on this information, we can, determine the coefficient of kinetic friction between cart B and the track. First, what is the mathematical relationship (that is, the general equation) between the friction force on B due to the track (f BT ), the normal force on B due to the track (N BT ), and the coefficient of friction,  ? How can we determine the magnitude of the normal force on B? Next, plug in the values for f BT , N BT , and solve for  . Show your work  Discuss your answers to section 3 with your lab instructor before continuing.

Lab: Interacting Bodies Physics 112 Copyright 2005 by S.E. Kanim, M.E. Loverude, & L.G. Ortiz 7 4.7: Consider the following discussion between three confused students. Alicia: “The force on B by A has to be greater in this case th an the force on A by B. If this were not true, then these forces would cancel, and system B would not accelerate.” Ben: “I agree. Since F=ma, and system A has more mass, there must be more force on it. So system A is pushing on system B with a greater force than system B is pushing on system A.” Callie: “The force of the hand is what’s moving the carts to the right. This force is the same for both carts, so they both have the same force on them to the right.” With which student statement(s), if any, do you disagree? Explain your reasoning. (Hint: If you disagree with any part of a statement, underline that part; do not assume any of the students are completely correct!) Create your own correct statement, to help these students understand the experiment: 4.8: You will now use Newton’s Laws to compar e the magnitudes of all of the horizontal forces that you identified on your free-body diagrams in part 4.2. This will be similar to Section 3.3 Write a Newton’s 3 rd Law equation comparing the magnitudes of the third law pairs you identified. (Hint: third law pairs are on different free-body diagrams.) Write a Newton’s 2 nd Law equation comparing the magnitudes of the horizontal forces on cart B. ( Hint: Recall that the carts are speeding up now.) Write a Newton’s 2 nd Law equation comparing the magnitudes of the horizontal forces on cart A. ( Hint: Recall that the carts are speeding up now.)

Lab: Interacting Bodies Physics 112 Copyright 2005 by S.E. Kanim, M.E. Loverude, & L.G. Ortiz 8 Of the horizontal forces on your free-body diagrams for carts A and B, which one is the largest? Explain. 4.9: How does a free-body diagram help us determine net force? Should a free-body diagram include an arrow for the net force? Challenge: Rank the magnitudes of all of the horizontal forces, from greatest to smallest.