Lab04-Forces-InPerson-labexploration

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Lab 4: Understanding Forces Physics 112 Lab Group Names: Copyright 2022 San Francisco State University; S.E. Kanim, M.E. Loverude, & L.G. Ortiz 1 1. Introduction In this laboratory, we look at the forces that are commonly encountered as we go about daily life: gravitational forces (weight), normal forces, tension forces, and friction forces. In order to find the net force acting on an object, we need to decide which of these forces are acting and to determine their direction. It is also useful to be able to reason qualitatively about these forces, and to understand what factors influence (a nd don’t influence) them. We will look at each of these forces in turn. The free-body diagram is a very useful tool in helping us identify and keep track of the forces acting on a body and of the directions in which these forces are acting. We will practice drawing free-body diagrams for situations involving the forces identified above. Equipment This lab uses force probes, which connect to your computer wirelessly. Generally, you will need to: • Launch the capstone software • Connect the force probe wireles sly and make sure capstone “sees” it • Choose an appropriate display as described in each section below Consult your instructor if you have difficulties.

Lab 4: Understanding Forces Physics 112 2 Copyright 2022 San Francisco State University 2. Free-body diagrams Two people are attempting to move a large block. The block, however, does not move. Chris is pushing on the block. Pam is pulling on a rope attached to the block. 2.1 Draw a large dot to represent the block. Draw vectors with their “tails” on the dot to show the forces exerted on the block. Label each vector and write a brief description of that force next to the vector. In Newtonian physics, all forces arise from an interaction between two objects. Forces are specified by identifying the object on which the force is exerted, and the object that is exerting the force. For example, in the situation above, a gravitational force is exerted on the block by the earth. 2.2 Describe the remaining forces you have indicated above in a similar fashion. The diagram you have drawn is called a free-body diagram . A free-body diagram should show only the forces exerted on the object or system of interest, that is, in this case, on the block. Check your free-body diagram and, if necessary, modify it accordingly. A proper free-body diagram should not have anything on it except a representation of the object and the (labeled) forces exerted on that object. A free-body diagram never includes (1) forces exerted by the object of interest on other objects or (2) sketches of other objects that exert forces on the object of interest. 2.3 All forces arise from interactions between objects, but the interactions can take different forms. Which of the forces exerted on the block require direct contact between the block and the object exerting the force? Which of the forces exerted on the block do not arise from direct contact between the block and the object exerting the force?

Lab 4: Understanding Forces Physics 112 Copyright 2022 San Francisco State University 3 We will call forces that depend on contact between two objects contact forces. We will call forces that do not arise from contact between two objects non-contact forces. 2.4 There are many different types of forces, including: friction ( ƒ  ), tension ( T  ), magnetic forces (F  mag ), normal forces (N  ), and the gravitational force ( W  , for weight). Categorize these forces according to whether they are contact or non-contact forces. Contact forces Non-contact forces 2.5 Consider the following discussion between two students. April: “I think the free -body diagram for the block should have a force by Chris, a force by the rope, and a force by Pam.” Bill: “I don’t think the diagram should show a force by Pam. People can’t exert forces on blocks without touching them.” With which student, if either, do you completely agree? Explain your reasoning. (Hint: underline any parts of a statement that you disagree with.) It is often useful to label forces in a way that makes clear (1) the type of force, (2) the object on which the force is exerted, and (3) the object exerting the force. For example, the gravitational force exerted on the block by the earth might be labeled W  BE . Your instructor will indicate the notation that you are to use. Label each of the forces on your free-body diagram in part 2.1 in the manner described above.  You may wish to discuss your answers above with your laboratory instructor before continuing.

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Lab 4: Understanding Forces Physics 112 4 Copyright 2022 San Francisco State University 3. Types of Forces In this section, we will be looking at the forces that are most commonly encountered in mechanics: weight, tension forces, normal forces, and friction forces. 3.1 Weight Hang the force probe vertically on the ring stand (so that the hook is hanging down) and turn it on. Connect the force probe wirelessly to the capstone software. “Zero” the force probe so that it will read zero when there is no force on the hook. To do this, click the properties icon and then button that says “zero sensor.” Consult your instructor if you have difficulties. A. Start recording data. Verify that the force is zero when nothing is touching the probe. Push and pull on the hook to observe the effect of exerting forces on the hook. (The force probe should read negative forces when the hook is pulled away from the body of the probe and read positive values when it is pushed toward the body of the probe.) B. Hang a 0.050-kg mass hanger from the force probe as shown*. After starting to record data, add slotted masses to the hanger until the total mass of the hanger and the slotted masses is 0.500 kg. (Use the table at right to record your data.) C. What is the mathematical relationship between the suspended mass and the force required to support it? [In other words, if you suspended a mass m from the force probe, how would you determine the force F that the force probe would read?] D. Is this relationship consistent with the mathematical relationship that you learned in the mini-lecture or in your book? * When we hang an object by the hook on the force probe so that it is at rest, the net force on that object is zero. Only two forces act on this object: the weight of the object and the force on the object by the hook. These two forces must have the same magnitude and act in opposite directions in order for the net force to be zero. For this reason, we can use the force probe to tell us about the weight of an object suspended from it. Suspended mass (kg) Force required to hold it (N). Force probe Slotted masses 0.050-kg hanger

Lab 4: Understanding Forces Physics 112 Copyright 2022 San Francisco State University 5 3.2 Tension forces, strings, and pulleys A. Connect a string between the mass and the force probe and repeat the experiment in part 3.1. B. What is the effect (if any) of the light string on the force measured by the force probe? C. Would the force measured by the probe change if you were to increase the length o f a light string? D. We can obtain a rough measure of the tension in a string by replacing portions of the string with some springs. The amount that a spring stretches increases as the tension increases. Connect two springs to force probes mounted horizontally on wooden carts, as shown. Zero the force probes before making any measurements. Predict which spring will have the greatest tension when you pull on the springs, and which will have the least. Explain the basis for your prediction. E. Test your prediction by sliding the force probes apart. (Since the springs are not perfectly identical, you should ignore any small differences in this and all subsequent exercises.) Resolve any inconsistencies. What do your observations suggest about the tension in different parts of a string or spring? Explain. Summarize what you have learned about the tension in a light string. Note that your conclusions about the tension are based on the experiments you did using light strings. If you were to use a heavy rope or a chain your conclusions would be different.  You may wish to discuss your answers above with your lab instructor before continuing. Force probe Force probe springs wooden cart wooden cart track

Lab 4: Understanding Forces Physics 112 6 Copyright 2022 San Francisco State University 3.3 Normal Forces In mathematics, a normal vector is defined as a vector pointing perpendicular to a surface (or a small portion of a curved surface as shown in the diagram). In general, when the surfaces of two objects are in contact with each other, these objects will exert forces on each other that are perpendicular to the surface of contact. In keeping with the mathematical definition of normal, these forces are called normal forces. A. For each of the following situations, identify (1) any normal forces acting on the identified object, (2) the objects that are exerting these normal forces, and (3) the direction of these normal forces:  A block sliding down an incline.  A marble resting in a v-shaped wedge.  A ladder leaning against a wall. B. Is the normal force always in the opposite direction to the weight? Explain.

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Lab 4: Understanding Forces Physics 112 Copyright 2022 San Francisco State University 7 Ask your lab instructor to exchange the hook on the force probe for a platform. Re-zero the force probe with the platform attached and the probe held vertically on a ring stand. C. Predict what value the force probe will measure with a 500-gram mass placed on the platform. D. Draw a free-body diagram for the mass. What force is exerted on the mass by the platform? Which force is the probe measuring? the normal force? the weight force? E. Now push down on the mass with your finger. Is the value measured by the force probe equal to the weight of the mass? F. Draw a free-body diagram for the mass with the finger pushing down on it. Based on this free-body diagram and the force measured by the force probe, what is the magnitude of the force exerted by the finger? Explain how you can tell. G. Remove your finger, and tilt the force probe with the weight on it as shown. (Be sure not to tilt the platform so far that the mass falls.) In this case, is the force exerted on the mass by the platform the same as the weight of the mass? H. Based on your answers above, under what conditions is the normal force exerted on an object equal to the weight of the object?  You may wish to discuss your answers above with your lab instructor before continuing. Free-body diagram for the mass

Lab 4: Understanding Forces Physics 112 8 Copyright 2022 San Francisco State University The normal force is a measure of how tightly two objects are squeezed together. As you saw when you pushed with your finger on the mass, the normal force between the mass and the platform increased – suggesting that the mass and the platform were squeezed more tightly together. 3.4: Friction Forces Attach the force probe with the hook to the string tied to the black plastic cart with the cork bottom surface. Zero the force probe before making any measurements. A. Place masses totaling 500 grams into the cart. Using the force probe, pull the string horizontally so that the cart moves along the track at a constant speed. Record the magnitude of the force measured by the probe. B Draw a single free-body diagram representing the forces acting on a system consisting of the cart and the masses in the cart. Include a key to the labels on your free-body diagram. C. Which force in your free-body diagram is the force probe measuring? Which other force in your free-body diagram has the same magnitude? Explain how you can tell. D. Repeat exercise A for the force carts with plastic and black fabric bottom surfaces with the same mass inside the cart. Is the friction force the same for all 3 carts? E. Add an additional 500 grams to the cart with the cork bottom, and again use the force probe to determine the friction force on the cart. F. Draw a single free-body diagram representing the forces acting on a system consisting of the cart and the masses in the cart for this situation. Identify the forces in your free-body diagram that are different from those in exercise A and those that are the same. Force probe wooden cart track

Lab 4: Understanding Forces Physics 112 Copyright 2022 San Francisco State University 9 G. Two students are discussing the friction force: Abel: The amount of friction acting on an object depends on the weight of the object. All other things being equal, it’s harder to push a heavier object. Beth: That’s true, but only because increasing the weight of an object increases how much the surfaces of the objects are squeezed together. Friction depends on the normal force. I. Based on your answers to exercises D-G do you agree with Abel or Beth? Explain. In introductory physics, we usually use a model for friction that is given by the equation f AB =  N AB , where  is called the coefficient of friction and N AB is the normal force acting on object A by object B. The coefficient of friction  is an experimentally determined quantity that depends on the surface of objects A and B that are in contact with each other. 4. Challenge Problem: Free-body diagrams again Now that we have looked at each of the forces (weight, tension, normal, and friction) individually, we finish this lab with a more challenging free-body diagram. A person stands on a ladder that is resting against a wall. Assume that there is no friction between the ladder and the wall but there is friction between the ladder and the floor. Draw a free-body diagram of the ladder in the space below. Label each force that you have included in your free-body diagram to indicate (1) the type of force, (2) the object on which the force is exerted, and (3) the object exerting the force. Include a key for your labels.

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