# Pendulum Stopper (Physics Magic)

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

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 Stopper trick, you’ll use the sympathetic motion of two pendulums to make energy appear to switch places . All you need are some household items and you’ll be fooling people in no time at all!

## 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 piece of string about 24 inches long,  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

Using their attached strings, tie each of the canisters 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.

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 and the process starts all over again!

## How Does It Work

When using two pendulums in this demonstration that alternate swinging, you are experiencing 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 pendulum that was initially stationary until the two pendulums have equal energy.  The energy continues to transfer away from the first swinging pendulum until it has completely stopped, and the second pendulum has all of the energy in full swing.  Soon, the process will start all over again as energy begins to transfer back to first pendulum.

## Take It Further

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.

## Science Fair Connection

Creating pendulums that share energy and alternate swinging is pretty cool, but it isn’t a science fair project.  You can create a science fair project by identifying a variable, or something that changes, in this experiment.  Let’s take a look at some of the variable options that might work.

• Change the lengths of the pendulum strings to see how long it takes for the energy to transfer from one pendulum to the other.
• Change the tautness of the connecting string to see how this affects the timing of the energy transfer.
• Try two pendulums of different lengths. Do they still share energy in the same way? Do they swing for equal amounts of time?
• Change the amounts of water in the film canisters. How does this affect how long each pendulum swings?

That’s just a couple of ideas, but you aren’t limited to those! Try coming up with different ideas of variables and give them a try.  Remember, you can only change one thing at a time.  If you are testing different lengths of string, make sure that she other factors are remaining the same.