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Magic with Physics - Pendulum Stop

Use principles of two pendulums in motion to play a fun trick on your friends

Rating: 43211

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While many magicians rely on the arts of deception or slight of hand to baffle their audience, we like to use principles of science to achieve the same effect. With the Pendulum Stop trick, you'll use the sympathetic motion of two pendulums to make your friends think they've developed telekinetic powers… or are a Jedi… we'll leave that part up to them. All you need are some household items and you'll be fooling people in no time at all!

Materials
  • Two sturdy stands
  • String or yarn
  • Two film canisters
  • Duct tape
  • Scissors
  • Water

Videos

  • Pendulum Stopper-Physics Magic - Sick Science! #099
Print Experiment

Experiment

  1. Carefully poke a hole in the lid of each of the film canisters using the scissors. A corkscrew or ice pick can also work great here.
  2. Cut two lengths of string that are about six inches in length and thread one through each of the film canister lids. Use duct tape to secure each string to the inside of the lids.
  3. Measure one tablespoon (?15 mL) of water into each of the film canisters and put the lids back onto the canisters.
  4. Cut a third, 24-inch, piece of string and tie it to both of the stands. Station the stands far enough apart that the string is fairly taut. Tape the knots tying the string to the stands so that the knots cannot slide.
  5. Tie each of the canisters, using their attached strings, to the string that connects the two stands. As you are tying the strings, try to make them as similar in length as possible.
  6. Slide the strings as necessary to space the canisters evenly as pendulums.
  7. Now that you have your dual pendulum apparatus, it's time for the magic! Pull one canister pendulum straight back and release it.
  8. As time passes, you begin to see the other canister pendulum (the one you didn't pull back) start to swing as well. The more this pendulum swings, the less the initial swinging pendulum swings until… WOW! The original swinging pendulum completely stops! Then the process starts all over again!
To turn this into a magic trick that convinces other people that they have telekinetic, Jedi-mind-trick type powers, tell them to focus on alternating pendulums. As they focus, the pendulum will react to their thoughts and slow until it stops. Then, tell them to release focus and the pendulum will begin swinging again! Finally, have them switch focus to the other pendulum as it starts all over again.

How Does It Work?

So you and your friends have clearly mastered the translation of your brain waves into measurable force, right? Well… no. Sorry. What you are experiencing is the sympathetic motion of two pendulums. As you pull back on one pendulum, you'll notice that you are also pulling back on the string that connects the two pendulums. The movement of the connecting string slowly begins to build energy in the initially stationary pendulum until the two pendulums have equal energy. The energy continues to transfer from the first swinging pendulum until it has completely stopped, and the process will start all over again.

Take It Further!

Experiment with different aspects of your dual pendulum apparatus to see what the effects are:

  • Change the lengths of the pendulums.
  • Change the tautness of the connecting string.
  • Try two pendulums of different lengths.
  • Change the amounts of water in the film canisters.

Customer Reviews

Simpler way to do this Review by Dr. S. Bettadpur
43211

Just use a pair of nuts instead of the film canisters; choose something large, like 1/2" or 3/4" size. Advantages: easier to tie (readymade hole!), smaller area => less damping!

Even simpler: Take a short length of PVC pipe (1" dia, about 6" long). Pass two threads through the pipe. To either end of one thread, attach nuts. Use the other thread to hold the pipe horizontally. You can adjust the lengths of the two pendulums by pulling on the weights, so you can investigate what happens when the pendulums are not resonant. Also, you can change the tension in the thread you are using to hold the pendulums.

(Posted on August 1, 2012)

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