Homemade Sciencewith Bruce Yeany

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Lesson plan · Grades 3-8

Can a string and two wheels make you stronger?

Students rig a small pulley system on a broomstick or dowel between two chairs and lift a half-full gallon jug of water, first with a single fixed pulley and then with a moving pulley (and, for older students, three strands). They measure the effort force with a spring or luggage scale and the length of cord pulled, and discover that pulleys trade force for distance.

Anchor phenomenon

Adding a second wheel to the rope makes the same jug feel about half as heavy. Where does the 'extra strength' come from, and what does it cost?

Learning objectives

  • Students will be able to measure the effort force needed to lift a load with a fixed pulley and with a moving pulley.
  • Students will be able to compare the distance the effort cord moves with the distance the load rises.
  • Students will be able to explain that a pulley system reduces force by increasing distance, and that mechanical advantage is about equal to the number of supporting strands.
  • Students will be able to calculate work in and work out and explain why they are not equal.

Lesson flow (5E)

Engage5 min

Have a volunteer lift the jug by hand, then with a single fixed pulley. Ask: did the pulley make it easier, or just change direction? Students predict what a second pulley will do.

Explore20 min

Groups weigh the jug, rig the fixed pulley and measure the effort, then add the moving pulley and measure again. For each set-up they mark the cord with tape and measure how far it moves for a 6-inch (15 cm) lift. Older groups add the 3-strand rig. Record results on the age worksheet.

Explain10 min

Groups share readings. Draw each rig and count the strands holding the moving pulley. Introduce mechanical advantage (load / effort) and the trade of force for distance; show the 2:00-2:30 section of Bruce's video where he balances one weight against two.

Elaborate5 min

Compute work in and work out for the two-pulley rig. Discuss where the difference goes (friction, the moving pulley's weight) and why spool pulleys lose more.

Evaluate5 min

Exit ticket: sketch a pulley rig with a mechanical advantage of 2, label the strands, and state how far you must pull to lift the load 10 cm.

Discussion questions

  1. If a single fixed pulley doesn't reduce the force, why do flagpoles and window blinds use one?
  2. How many strands were holding up the jug in each set-up? How does that compare with your scale readings?
  3. Why did you have to pull more cord when the force got smaller?
  4. Why was your measured mechanical advantage lower than the strand count?
  5. Can a pulley system ever give you more work out than you put in? Why not?
  6. In the video, why does the principal need less force when he pulls the rope himself?

Common misconceptions

Common ideaPulleys make energy or reduce the work needed.What the demonstration showsThey reduce the force but increase the distance; work in is equal to or (with friction) more than work out.
Common ideaAny pulley makes lifting easier.What the demonstration showsA single fixed pulley only changes the direction of the force; its mechanical advantage is 1.
Common ideaMechanical advantage equals the number of pulleys.What the demonstration showsIt equals the number of strands supporting the moving pulley; two pulleys can give 2 or 3 depending on how they're rigged.
Common ideaThe scale should read exactly half with two strands.What the demonstration showsFriction and the moving pulley's weight make it read a bit more than half.

Assessment

  • Completed data table with effort force and cord pulled for each set-up, from the age-band worksheet.
  • Exit ticket: diagram of a mechanical-advantage-2 rig with strands labeled and the pull distance for a 10 cm lift.
  • Performance task: rig the pulleys to make the scale read about one-third of the load, and explain the strand count.
  • Short written explanation of why work in is greater than work out.

Differentiation

Support: Pre-rig the fixed pulley and have students only add the moving pulley; use a feel test (easier/harder) and a simple tally of cord length instead of scale readings.

Extend: Measure in newtons and meters, calculate efficiency for each set-up, compare spool and store pulleys, and design a rig for a mechanical advantage of 4 using borrowed double pulleys.

Classroom safety. Use small loads only (a half-full gallon jug or a few books) and lift them only a few inches. Weight the chairs so they can't tip, keep feet out from under the load, and do not allow anyone to hang from the bar or to rig the system to lift a person; the people-lifting in the video used a ceiling anchor and close teacher supervision and is not part of this lesson.