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Drop of Doom Inquiry

In a simple one-pulley system, the heavier weight will move down and pull the lighter weight up. 

When the lighter weight is held out at an angle, it spins around the pulley (just a stick, here) and holds the heavier weight up.

  Why?

Drop of Doom

First, consider the heavier weight. It falls straight down because the force of gravity pulling down is stronger than the tension in the rope pulling up.

The lighter weight behaves like a pendulum, swinging down constrained by the rope in such a way as to convert potential to kinetic energy.

Now, the fascinating issue is that the pendulum gets shorter as it swings. Conservation of angular momentum means that as you make the rope shorter the little weight moves faster. At some point it moves fast enough that it will make a complete rotation around the bar.

The last consideration is the friction between the rope and the bar. After a sufficient number of turns, there’s enough friction to support the heavy weight. The tension required to hold the weight decreases exponentially with the amount of wrap, so that 5 wraps will allow a 300,000-fold difference between the small weight and the large one.

Although a full analysis of the motion is beyond the scope of high school physics, there is a lot of potential for inquiry in this activity, even at the elementary school level, to help students explore and understand the forces and motion involved.

  • Vary the length of string
  • vary the weight on each end of the string
  • vary the angle of the pendulum as it’s dropped
  • vary the kind of string

Objectives

  • Generate testable questions about a surprising demonstration.

  • Design simple experiments to uncover the “rules” governing the demonstration.

Materials

  • Per Drop of Doom apparatus:
    Wine glass
    1m of butcher twine or cotton string
    House key or washer
    pencil

  • Per Investigating Group:
    1m length of string (or more)
    Scissors
    At least 12 identical washers—about 1 inch diameter (as long as they’re all the same the size doesn’t matter much)
    Pencil

  • Extra inquiry equipment:
    Different pencils or dowels (round, multisided, thick, thin)
    Different kinds of string (thin, thick, stretchy, rough, smooth)

Key Questions

  • What could happen when I let go of the key?
  • What is important about the equipment to make the trick work?

What To Do

Introduction: Demonstrate the Drop of Doom

Teacher Tip: Practice this over a pillow before you perform for an audience.

  1. Have a student verify that the wine glass is heavier than the key.
  2. Hold the string draped over the pencil so the wine glass hangs down on one side and the key on the other.  Ask, “What could happen if I let go of the key?”
  3. Show that the wine glass will fall and pull the key up.
  4. Now hold the string at a 90 degree angle and repeat the question and the demonstration.  The key will swing around the pencil, wrapping itself tight and preventing the glass from falling.

Inquiry

  1. What questions do you have?  Brainstorm a list. For less experienced scientists, you can scaffold with a sentence starter like, “What would happen if…” or even ask directly, “What might be important to make this work?”
  2. Show students how to create a safe version of the apparatus by tying a bunch of washers on one end of a string and one washer on the other end.
  3. Easy independent variables to change are: the length of the string, the position of the pencil along the string, the total weight of the items on each end, the difference between the weights on each end, the size/shape/slipperiness of the pencil.
  4. Have groups of students pick a question and create an experiment to test it. The goal is not to develop a comprehensive “explanation,” but to develop some repeatable cause and effect rules for this phenomenon.

Extensions

Other Resources

Science World | YouTube | Exploring the Conservation of Energy (Pendulum Fun!)

Catalyst for Science | John Eix | Explanations of Discrepant Events