Key Concepts
Physics
Energy
Potential energy
Conservation of energy

Introduction
If you enjoy chain reactions and spectacular sights, this activity is for you! Can you imagine how weaving rather unspectacular tongue depressors (or popsicle sticks or craft sticks) could set off a chain reaction of sticks flying in the air? Try it out and see for yourself!

Background
Energy is never created; it always gets transferred from one kind to another. This activity is a great example of this key physics principle!

If you have stretched a rubber band, you have felt how it wants to come back to its original length. Scientists say that a stretched rubber band is loaded with elastic potential energy. In a similar way a wooden tongue-depressor stick (like the kind used at the doctor's office) stores elastic potential energy when you bend it (though not if you break it). When you allow it to go back to its flat state it releases this energy.

If a bent depressor stick straightens up near a hard floor, the stick might push itself off of the floor and fly into the air. The elastic potential energy that was stored in the stick is transformed into movement, called kinetic energy. As it flies up it gains height above the floor, which translates into gravitational potential energy, or the energy due to the fact that it can fall back down. Eventually the stick lays flat on the ground. The energy once stored in the stick has ultimately been transformed into heat and sound—other forms of energy.

When one tongue depressor stick releases its stored energy nothing too spectacular happens, but what will happen when a chain of sticks release their energy?

Materials

• At least 50 tongue depressors. If these are not available, large, somewhat flexible popsicle sticks or wide craft sticks are a good alternative.
• Hard floor
• Four markers: light blue, dark blue, orange and red
• Metric ruler
• Glue
• Pen (optional)
• Video camera or device that takes video (optional)

Preparation

• Color four tongue depressors light blue, four dark blue, four orange and seven red.
• Make a piece to start your stick chain by placing one light blue, one dark blue and another light blue tongue depressor parallel to each other. Leave about two centimeters between the depressors.
• Stagger the parallel depressors so each depressor sticks out two to three centimeters above the adjacent one.
• Place a red depressor at a right angle to all three depressors. Place it on top of the three depressors, and adjust it until it just touches all three of them. Glue it in place.
• Place a second red depressor next to the first red one, and glue it to all three staggered depressors. Let the glue dry.
• Turn the structure over, and glue two red depressors matching the ones of the other side. Now the three blue staggered depressors will be sandwiched between the red ones. Allow the glue to dry.
• The light blue depressor that sticks out most will be "1" (you can use a pen to label it if you like). The middle (dark blue) depressor will be "2," and the one that sticks out the least (light blue) will be "3."

Procedure

• To start your chain gather the starter piece that you assembled (ensuring that the glue is dry), the few colored tongue depressors that are left and a large number of uncolored tongue depressors on your hard floor.
• Slip an orange depressor in the starter piece so it goes under depressor 1 (light blue), over depressor 2 (dark blue) and under depressor 3 (light blue). It should stick out from under depressor 3 just a little. Shift it until it is parallel to the red depressors from the starter piece, leaving about two centimeters between them.
• Slip a red depressor in the starter piece so it goes over depressor 1 (light blue), under depressor 2 (dark blue) and sticks out from under depressor 2 just a little. Shift it until it is parallel to the orange depressor, leaving about two centimeters between them.
• You should see two parallel depressors (the orange and the red depressors you just added) sticking out. Slip a dark blue depressor under the shorter one (the orange one) until it sticks out a little and over the longer one (the red one), parallel to depressor 1 (light blue). You might feel you have to hold the depressors in place. Why would this be?
• You should now see two parallel depressors sticking out: a light blue and a dark blue one. Slip an orange depressor under the shorter one (light blue) until it sticks out a little and over the longer one (dark blue) so it makes a right angle with the sticks it crosses.
• Find the two parallel depressors sticking out: they are red and orange. Slip a light blue depressor under the shorter one until it sticks out a little and over the longer one so it makes a right angle with the depressors it crosses. Do you notice that all the blue depressors are parallel, and that all the red and orange depressors are parallel?
• Find the two parallel depressors sticking out: they will be at a right angle with the last depressor you added. Slip a depressor under the shorter of the two until it sticks out a little and over the longer one so it makes a right angle with the depressors it crosses.
• Repeat previous step multiple times. Do you feel like you have to bend the depressors? How would that help create an energetic chain reaction? What do you think will happen when you no longer hold down the last couple of depressors you weaved into the chain?
• Keep building until you are satisfied with the size of your chain. You can keep the depressors in place by fixing the last depressor you are adding at a diagonal under the one that would be parallel to it if you were continuing making the chain longer.
• When you are ready to let your chain reaction go, watch closely, and let the last depressor go. What happens? Why would this happen? Did you notice other forms of energy as the reaction occurs?
• Extra: Build a longer chain and see how the chain reaction changes as the chain gets longer.
• Extra: If you have a video camera (or phone or other device that takes video) available, take a video of the chain going off, preferably in slow motion. What can you learn from the slow-motion video?
• Extra: For a challenge make your chain bend or make it go over obstacles.
• Extra: Compare the chain reaction on carpet to a similar chain on a hard floor. Which one makes the sticks jump up highest? Why would this be the case?
• Extra: Make chains from extra-large craft sticks and tongue depressors. Can you explain why the result is different?

Observations and Results
You probably saw the tongue depressor sticks jump up in the air as soon as you released the last one—and maybe even before you planned to let them go.

You might have noticed that you had to bend the depressors in order to make them fit into the chain. With each bend you added energy to the system. Did you feel how the chain was under tension and how you had to hold the last couple of depressors to keep them in place? When you let it go all this tension is released in a chain reaction. It starts with the last depressors you added straightening. This releases the next depressors in the chain and so on. Soon all the depressor sticks fly in the air, releasing the energy stored in them—and you have a whirlwind of sticks flying around!

If you assembled your chain and left it out for a day or so, you might have noticed your depressors did not jump as high when you released the chain. Over time the depressors yield under the tension and become permanently bent. Part of the energy goes into bending the material (usually wood).

If you tried to release the chain on carpet or any other soft ground, you might also have noticed the depressors did not jump as high. The depressors cannot bounce off the soft ground as easily as they can off of hard flooring because the softer ground absorbs part of their energy.

Maybe you noticed it is harder to weave shorter or more rigid popsicle sticks together. These sticks do not bend as easily; instead they tend to break. They are not as well suited to store elastic potential energy

More to Explore
Popsicle Stick Chain Reaction—STEM Activity, from Science Buddies
Build a Paper Roller Coaster, from Science Buddies
STEMonstrations: Kinetic and Potential Energy, from NASA
STEM Activities for Kids, from Science Buddies

This activity brought to you in partnership with Science Buddies