Comparing Mechanical Advantage and Efficiency of Pulleys

Shelly Leatham, Chanhassen High School, Chanhassen, MN
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Summary

This lab activity is designed to allow students to experience what an increase in mechanical advantage means. Students determine the mechanical advantage of three pulley set-ups. Students also measure the work input and output, then calculate the efficiency. Finally, students determine the relationship between the mechanical advantage and the efficiency of the pulleys.

Learning Goals

The main goal of this activity is for students to experience how work is made easier by the use of various pulleys. Students will need to use data analysis and critical thinking to complete this lab activity. KEY CONCEPTS: Students should learn that pulleys make work easier by multiplying the input force over a longer distance. Students should also learn that efficiency is a comparison of work input to work output. Finally, students should understand that the efficiency of a pulley (or pulley system) decreases as the mechanical advantage of the machine increases.
VOCABULARY WORDS: Mechanical advantage, work input, work output, efficiency

Context for Use

This laboratory activity is appropriate for an introductory physics course at around a ninth-grade level. It works best when students can work in groups of two to three. It can be completed, including instruction on how to do the lab, in about 40 minutes. Students should have an understanding of why we use simple machines and a basic understanding of mechanical advantage and efficiency, though these concepts will likely be better understood upon completion of this activity. I usually use this as my first lab activity in my simple machines unit because of its simplicity: students can complete it easily and can literally feel the difference in how much easier the work is as the mechanical advantage increases.

Description and Teaching Materials

Student Laboratory Sheet for Pulley Lab (Microsoft Word 42kB Aug1 09)

.doc (Microsoft Word 44kB Aug4 09)

Materials (per group):

  • two to four single pulleys
  • cotton string (see below)
  • 500 g weight with a hook
  • 5 N spring scale
  • one to two meter sticks
  • laboratory activity handout
  • EITHER two upright bars, one crossbar and two clamps OR one to three ring stands with attached rings

Teacher set-up (per group):

  • cut two to three cotton strings for each group - one for fixed pulley (same can also be used for movable) and one longer sting attached to a pulley for the block and tackle
  • cut one smaller piece of cotton string - tie into loop on one end of single pulley to hang pulley

Teaching Notes and Tips

I like to set-up examples of each pulley system at the front of the classroom so A) I can show students beforehand what they will measure during lab and B) can then leave the examples for them to view to ensure their set-ups are correct. If the set-ups hang in front of a white board, writing can be left on the board to represent which measurements are made and where they are written on the handout.

Assessment

My preference is to informally assess each group with questions as they work. I stop by each group to check in and ask them what they expect to collect for data, or if they can explain why their data are as they found. I also usually print a set of questions on the back

References and Resources

Student Laboratory Sheet for Pulley Lab (Microsoft Word 42kB Aug1 09)

.doc (Microsoft Word 44kB Aug4 09)