Wednesday, November 16, 2011

AP Physics:
How Do Pulleys (Do) Work?

Learning Objectives
Students will explore the significance of work by experimenting with pulleys.

Assessment Type
This activity is meant to partially replace a traditional lecture on the topic of work. It can be used as a brief hands-on laboratory activity, or it can be completed using the provided simulation in an online or hybrid classroom. The purpose of the activity is to generate class discussion and is best left ungraded.

Assignment Details
  • Work is a difficult concept for students to understand, even the second or third time they encounter it. Simple machines in general, and the pulley in particular, provide excellent tools for exploring work since simple machines allow us to trade force for distance to transfer the same amount of energy. 
  • Begin the lesson by introducing the concept of work and providing students with the equation for work.
  • Without making any apparent connection to work, introduce students to the pulley. This can be accomplished with a set of physical pulley systems around the classroom (1-, 2-, and 4-pulley systems would work best). Alternatively, you can use the pulley gizmo with its 5-minute free trial. 
  • Do not over-introduce how pulleys work - simply point out that pulleys can make it easier to lift heavy objects, and that the more pulleys there are, the easier it is to lift the object.
    • Pose this question to students: How can simply adding pulleys make something lighter?
    • Ask students to work in teams of 2-3 to investigate this question and write out their explanation (including a drawing) on a white board or sheet of paper. 
    • When students are complete, ask a few groups to share their work with the class and discuss.

    Instructor's Notes
    • To make the investigation more quantitative, provide students with force meters and meter sticks. Students should be able to show that the same amount of work is done with each pulley configuration. 
    • A good transfer of knowledge question (in class, on homework, or on a test) would be to ask why pushing a cart up a ramp is 'easier' than lifting it straight up. You could follow up by asking whether we measure the difficulty of a task by the amount of work required or by the force required (the answer is not trivial).  


    Laura Bost said...

    Yay, fun with pulleys! I wanted to comment on the question you suggested we ask students, "How can pulleys make things lighter?" I suggest the follow-up question of, "Is the object really lighter?" After all, the object's weight hasn't changed. This leads into a fun discussion the forces in play when pulleys are used.

    Andrew Vanden Heuvel said...

    Hi Laura, great suggestion! Maybe we could even ask students to determine the weight of an object that is too heavy to measure with a typical spring scale. Pulleys can be one of the tools at their disposal. Also we could ask students to compare the weight of an object and the force applied to the pulley in an effort to see how much friction the pulleys are adding to the system. Thanks for posting!

    Andrew Vanden Heuvel said...
    This comment has been removed by the author.