θ Beam Pivot F r Figure 1. Figure 2. STATICS (Force Vectors, Tension & Torque) MBL-32 (Ver. 3/20/2006) Name: Lab Partner: Lab Partner:

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1 Please Circle Your Lab day: M T W T F Name: Lab Partner: Lab Partner: Project #1: Kinesthetic experiences with force vectors and torque. Project #2: How does torque depend on the lever arm? Project #1: Experiencing vector forces. Prediction #1 1. Examine the situation shown in Figure 1. Does the direction shown for F r seem to you to be reasonable? F r is the force exerted on the beam by the assembly. Circle your answer: YES NO Explain why you believe your answer? F r Guy wire θ Beam Pivot Figure 1. W Prediction #2 2. Examine the situation shown in Figure 2. Does the direction shown for F r seem to you to be reasonable? F r is the force exerted on the beam by the assembly. Circle your answer: YES NO Explain why you believe your answer? F r Pivot Guy wire Figure 2. Beam W (Project#2 after J. Knerr -- Physics 251L) 1

2 3. Each person in your group pick up, hold by its hook, and experience how much force must be exerted to support a 200 g mass. 4. Use a meter stick, two meter stick clamps, string, and a 200 g mass to simulate Figure 2. Place the clamp that will hold the 200 g mass at the 90 cm mark and the clamp for the vertical string at the 20 cm mark as shown in the photograph (simulation of Figure 2). Each person in the group take turns holding the stick at position (A) and the string at position (B) all the while making sure the stick is horizontal and the string is vertical. (A) (B) Figure 3 a) Give here a clear description of the direction of the force you exerted at position (A) and draw a force vector that represents the force you exerted at (A) on the photograph. Does this agree with your Prediction #2? If not, explain why you think your prediction was incorrect. b) Give here a clear description of the direction of the force you exerted at position (B) and draw a force vector that represents the force you exerted at (B) on the photograph. c) How does the magnitude of the force you exerted at (A) compare to the force required to d) How does the magnitude of the force you exerted at (B) compare to the force required to (Project#2 after J. Knerr -- Physics 251L) 2

3 5. Move the string clamp to the 40 cm mark so that the arrangement is similar to this photograph so that you can simulate Figure 1. Make an angle θ < 40 o. Each person in the group take turns holding the stick at position (A) and the string at position (B) all the while making sure the stick is horizontal and the string angle is the same. STATICS (Force Vectors, Tension & Torque) MBL-32 (A) (B) θ Figure 4 a) Give here a clear description of the direction of the force you exerted at position (A) and draw a force vector that represents the force you exerted at (A) on the photograph. Does this agree with your Prediction #1? If not, explain why you think your prediction was incorrect. b) Give here a clear description of the direction of the force you exerted at position (B) and draw a force vector that represents the force you exerted at (B) on the photograph. c) How does the magnitude of the force you exerted at (A) compare to the force required to d) How does the magnitude of the force you exerted at (B) compare to the force required to (Project#2 after J. Knerr -- Physics 251L) 3

4 Project #2: How does torque depend on the lever arm? In the following setup, the string tied to the force probe prevents the bar from rotating around the cylindrical support. force probe peg cylindrical support bar 1. Predict how you think the tension in the string (measured by the force probe) will change as the peg is moved away from the cylindrical support. Please try to explain (in words, not equations) your prediction. Make sure the force probe and the rod are horizontal and the string is vertical. 2. Do the experiment (remember to calibrate the force probe). Make a force vs. time graph. Every few seconds, switch the peg to a different hole on the bar (farther from the cylindrical support). Sketch and label your graph below. Print a copy for each member of the group. Q: Was your prediction correct? If not, what about it was wrong? (Project#2 after J. Knerr -- Physics 251L) 4

5 3. Derive an algebraic expression (no numbers, yet) for the tension in the string needed to keep the system in static equilibrium when the peg is in hole #18 (away from the support). Hint: Σ torques about the cylindrical support = 0, and there are 3 torques. 4. Measure the masses of the bar and the peg: m bar = kg m peg = kg 5. Measure the lever arms for each torque: r bar = m r peg = m r string = m 6. Use your equation from #3 to calculate the tension in the string for the peg in hole #18: T = N (calculated) 7. Measure the tension in the string (with LoggerPro and the Force Probe): T = N (measured) 8. What is your percent difference between the calculated and the measured values? Is this difference less than the expected experimental error due to uncertainties in the measured quantities? (Project#2 after J. Knerr -- Physics 251L) 5

6 Homework: H1. Based on what you have learned about forces, torques, and static equilibrium, indicate on this figure a reasonably appropriate vector for F r. Explain why you think your vector is reasonable. Pivot Guy wire θ Beam W H2. Consider the drawing at the beginning of Project #2. Suppose the force probe reading is 2.13 N. If the force probe and its string were moved so that the string is attached to the bar at a distance of 1/3 that shown in the drawing from the cylindrical support, what would be the reading of the force probe? (This would be similar to Figure 2 of Project #1.) Force probe reading Explain why you believe your answer is correct. H3. Continuing from H2, suppose the force probe's string is still attached to the 1/3 point, but now the string makes an angle of 70 o with respect to the rod (similar to Figure 1 of Project #1.) Will the tension in the string be greater than, the same as, or less than when the string was vertical? Circle your answer. greater than same as less than Explain why you believe your answer is correct. H4. Can a small force produce a larger torque than a large force? If not, why? If yes, give an example. (Project#2 after J. Knerr -- Physics 251L) 6

7 H5. Suppose the maximum tension the string can support is 100 N. What is the maximum peg mass the string can support in hole #18? (Project#2 after J. Knerr -- Physics 251L) 7