You work for NASA designing a probe to search for new life on other planets. The probe is deployed from a lander after it touches down on the planet being explored. The probe sensor needs to be moved a short distance from the lander after touchdown, and you are looking for a simple and inexpensive mechanism to accomplish this. One of your colleagues suggests rolling the sensor away from the lander on a single spool-like wheel. Your colleague believes that by pulling on a cord wrapped around the spool, it can be made to roll while the cord is unwinding. The ground where the lander will be located will be rough, so the spool will roll but won’t “slide.” You are not certain your colleague’s idea will work. Another colleague suggests that the angle of the cord will affect how well this method works. You decide to build a model of the situation in lab to determine if pulling on a cord wrapped around a spool will make it roll, and whether the angle of the pull is significant.
Instructions: Before lab, read the laboratory in its entirety as well as the required reading in the textbook. In your lab notebook, respond to the warm up questions and derive a specific prediction for the outcome of the lab. During lab, compare your warm up responses and prediction in your group. Then, work through the exploration, measurement, analysis, and conclusion sections in sequence, keeping a record of your findings in your lab notebook. It is often useful to use Excel to perform data analysis, rather than doing it by hand.
Read Sternheim & Kane: sections 5.1-5.4. Review sections 1.3-1.6, 2.1-2.4, 3.1, 3.3-3.10, 4.1-4.2, 4.10.
Equipment
The equipment for this lab consists of a spool and string. Also supplied is a meterstick. /If equipment is missing or broken, submit a problem report by sending an email to . Include the room number and brief description of the problem.
Warm up
1.Sketch a diagram of the spool and draw all of the forces acting on it. Indicate forces that cause a torque on the spool. Write an expression for the total torque acting on the spool about the spool center.
2.Is the angle that the string is pulled present in your total torque equation? Which direction will the total torque cause the spool to roll? Does the angle the string is pulled at affect which direction the spool will roll? Is there any special angle at which the behavior changes? Do your calculations make any approximations?
3.While pulling on the string will the spool be accelerating? Describe the motion of the spool.
Prediction
Will pulling on a string wrapped around the center of a spool resting on the ground (as pictured) cause the spool to roll? Assume that the point of contact between the spool and the ground or table is “rough” so that the spool won’t slide. Does the angle that the string is pulled at affect whether the spool rolls or what direction it rolls? If so, how does the angle matter?
Exploration
Set up the system as shown in the equipment section. Try pulling on the string to see which direction the spool moves. Try pulling the string at different angles to see if the string angle influences the behavior. Explore all possible angles (there is slightly less than 180 degrees possible). Pull on the string with steady pressure, not with sharp tugs.
Measurement
Record which direction the spool rolls when you pull on the string at specific angles. Determine whether the behavior changes at a particular angle, and whether this agrees with your predictions. Can you explain the behavior of the spool in terms of torque?
Analysis
Can you explain the behavior of the spool in terms of torque? Did the behavior of the pool change when you pulled at a particular angle? If so, what angle was this and why did the behavior change? Explain how the different forces acting on the spool work together to cause the behavior you observed.
Conclusion
How did the system behavior agree with your predictions? Did you make any approximations in your calculations that might affect your prediction accuracy? Explain the behavior of the system in terms of torque.