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Forces Laboratory. After Completing This Lab and the Associated 1.- Forces laboratory. Lab 4: Forces Name ______________________ 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 (and don’t influence) them. We will look at each of these forces in turn. Newton’s second law states that the net force acting on a body is equal to its mass times its r r acceleration: Σ F = ma . The acceleration of an object is a vector quantity, and we must take the direction of this quantity into consideration as well as the magnitude. Likewise, force is also a vector quantity. 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 so that we can find the net force. Free-body diagrams serve as a guide for adding these forces to find the net force. In this lab, we will practice drawing free-body diagrams for situations involving the forces identified above. We will use a convention for labeling these forces that serves to minimize common errors and that will also help to identify Newton’s 3 rd law pairs. 1.1 Lab Objectives. After completing this lab and the associated homework, you should be able to: 1. Identify the forces acting on an object for typical situations encountered in mechanics. 2. Draw a free-body diagram that includes identification of the type of force, the object on which the force is acting, and the object exerting the force. 3. Relate the mass of an object to its weight 4. Predict the direction of tension forces and the effect of pulleys on tension forces. 5. Identify normal forces acting on an object. 6. Correctly identify friction forces acting on an object. 1 1.2 Outline of Laboratory Approximate sequence of the lab and homework: 1. Practice identifying forces and labeling them in a systematic manner. 2. Relate the mass and weight of objects. 3. Use springs to observe the magnitudes of forces along a string. 4. Observe the effect of pulleys on forces. 5. Practice correctly identifying the direction of the normal force. 6. Find out which factors do and do not affect the magnitude of the friction force. 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. Copy your group’s sketch here after discussion. Chris Pam 2.1 Draw a large dot on your large sheet of paper 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. 2 Sometimes a free-body diagram involves a simplified sketch of the object rather than the dot. (Your instructor will indicate the convention you are to use.) Regardless of which form is used, 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 Write your observations and comments about this point of the lab. 3 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? 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-contactr forces. r 2.3.1 There are manyr different types of forces,r including: friction ( ƒ ), tensionr (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 2.4 Consider the following discussion between two students. Student 1: “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.” Student 2: “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 agree? Explain your reasoning. 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 rthe force. For example, the gravitational force exerted on the block by the earth might be labeled WBE . 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. Discuss your answers above with your laboratory instructor before continuing. 4 ➪ 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 Hold the force probe vertically (so that the hook is hanging down). Your lab instructor will show you how to ‘zero’ the force probe so that it reads zero when there is no force on the hook. (The force probe will read negative forces when the hook is pulled away from the body of the probe and will read positive values when it is pushed toward the body of the probe.) 3.1.1 Start recording data and push and pull on the hook to observe the effect of exerting forces on the hook. 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: Force probe 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 Slotted masses in order for the net force to be zero. For this 50-g hanger reason, we can use the force probe to tell us about the weight of an object suspended from it. 3.1.2 Hang a 50-gram 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 500 grams. 3.1.3 What is the 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? 3.1.4 Is this relationship consistent with the mathematical relationship between force and weight (on the surface of the earth) that you have used in the lecture portion of your physics course? 3.2: Tension forces, strings, and pulleys 5 3.2.1. Replace the hook on the force probe. Connect a string between the mass and the force probe and repeat the experiment in part 3.1. 3.2.2. What is the effect (if any) of the string on the force measured by the force probe? Force probe 3.2.3 Suppose instead that the mass were connected to force probe by a 1-kg chain. Would you expect the force measured by the probe to change? Explain. Under what condition(s) can we ignore the mass of a string (thread, rope, wire, or chain) when measuring tension forces? Slotted 3.2.4 Would the force measured by the probe change if 50-g hanger masses you were to increase the length ⋅ Of a light string? ⋅ Of a heavy chain? 3.2.5 We can obtain a rough measure of the tension in a string by replacing portions of the string with springs. The amount that the spring stretches increases as the tension increases. Rod Hang a string with three springs from a hook on a rod as shown in the diagram at right. Hang the hook without any slotted masses from the lower loop on the string. Spring 1 When slotted masses are added to the string, the springs will stretch.
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