A yo-yo is a working lesson in potential energy, kinetic energy, and friction. You will need: one yo-yo, a meter stick or tape measure, and a stopwatch (a phone works).
1. You hold the yo-yo 1.0 m above the floor, ready to throw. Calculate its gravitational potential energy.
2. You hold the yo-yo only 0.60 m above the floor. Calculate its potential energy now. How does it compare to your answer in #1?
3. You release the yo-yo from 1.0 m. Ignoring friction, all of its potential energy becomes kinetic energy by the time it reaches the bottom. What is its kinetic energy at the bottom? Explain why in one sentence.
4. Challenge: A yo-yo is dropped (not thrown) from 0.80 m. Ignoring friction, how fast is it moving when it reaches the bottom? Hint: set KE = PE, then solve KE = ½mv² for v: v = √(2·g·h).
Procedure: (1) Measure your full string length. (2) Throw 10 sleepers and time each with a stopwatch; record below. (3) Shorten the string by about 10 cm (wrap it around the axle a few extra times or tie a knot) and repeat. (4) Shorten by another 10 cm and repeat a third time.
| String length | Sleeper times (seconds) | Average | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Full: ____ cm | |||||||||||
| −10 cm: ____ cm | |||||||||||
| −20 cm: ____ cm | |||||||||||
Graph: sketch a bar chart of average sleep time vs. string length.
5. In a perfect, frictionless world your yo-yo would return with 100% of its energy. In reality it doesn't. Where does the "missing" energy go? Name at least two places.
6. Look at your graph. Did shorter strings sleep longer or shorter, or was there no pattern? Propose one physical reason for what you observed.
| 1. 0.64 J (0.065 × 9.8 × 1.0 = 0.637 ≈ 0.64 J) | 2. 0.38 J (0.065 × 9.8 × 0.60 = 0.382 ≈ 0.38 J — about 60% of #1, since PE is proportional to height) |
| 3. 0.64 J (energy is conserved — ignoring friction, all PE becomes KE) | 4. ≈ 4.0 m/s (v = √(2 × 9.8 × 0.80) = √15.68 ≈ 3.96 m/s) |
5. Accept: friction between string and axle/bearing → heat; air resistance → heat; sound energy; some energy remains as spin ("sleep") instead of returning upward. 6. Open-ended — look for a claim tied to the student's data plus a plausible mechanism (e.g., shorter string = less string rubbing in the gap = less friction; or shorter string = harder clean throw = more wobble).