Extreme Science: Penny Shine

Join us every Saturday morning for Extreme Science with Radical Rick.

This week Rick shows us the "Pendulum Drop".

Here's what you need for this experiment:

  1. Shoestring or similar type and length of string or yarn
  2. Scissors
  3. 16 medium size hex nuts or washers
  4. Pencil
  5. Adult supervision


  1. Tie one hex nut (or washer) to one end of your string.
  2. Make a 2-3" loop in the other end of your string and tie this off
  3. Thread your string (looped end) through all but one of your remaining hex nuts.
  4. Thread the looped end of the string through the last hex nut so that the loop is sticking out about 2 inches. Open up the loop and pull this hex nut through the loop and pull the string tight to secure it in place.
  5. Hold your index finger (or a pencil) out and place the string over your extended index finger.
  6. Holding onto the single hex nut, pull the string horizontally bringing the large group of hex nuts up so that they are almost touching your finger.
  7. With the single hex nut being held level with your extended index finger, let go of the end with the single hex nut and watch what happens!

What is going on?

A pendulum is a weight that is suspended from a fixed point that swings back and forth due to the force of gravity. When you release the single hex nut, common sense tells us that, since the other end of the string is so much heavier, gravity will pull it towards the floor pulling the single hex nut over the top of your finger and to the floor below. We must remember that gravity is also pulling on the single hex nut causing it to accelerate towards the ground when we release it. Since the length of the string between your finger and the single hex nut is rapidly decreasing the velocity of your pendulum increases which in turn increases the amplitude causing the string to wrap around your finger several times.

Try this:

  • Try adding more hex nuts to the heavier end of the string. You can easily do this by pulling the loop back over the single hex nut, string more hex nuts on and loop the string around the last one securing it in place again.
  • How much weight can you add to the heavy end and still have it work?
  • How many can you take off and still have it work? Would this work with something as light as a feather on the light end of the string?
  • What happens if you change the length of the string?

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