 Resonance is all around us, from musical instruments to playground swings, and even large structures such as skyscrapers and bridges. Resonance can occur just about anywhere. In some cases, like the swing, resonance can be a positive quality. If someone pushes you on a swing at just the right speed in sync with its natural frequency, it propels you higher. But in some cases, resonance can be detrimental. Take the 1940 Tacoma Narrows bridge collapse, for example. Before its collapse, the bridge experienced large amplitude oscillations when the wind was blowing. The collapse occurred when the wind generated flutter in the structure, which matched the structure's natural frequency, causing large amplitude oscillations and stressing the bridge beyond its design strength. All materials have a natural frequency, and thus can experience resonance. But where does this property originate? Is it a material property? Is it independent of material and a function of the structure? These are all interesting questions, so the natural conclusion is that we need, you guessed it, a hypothesis. I know that all materials exhibit a natural frequency and have a resonance frequency. If you look very closely at a few skyscrapers, you might see them swaying in the wind a bit. Tall buildings and bridges are mostly made of the same materials, so they must have the same natural frequency. Therefore, I will make the following hypothesis. Natural frequency is a material property and is not influenced by design. So now we'll need an experiment to test our hypothesis. A good way to test if something is material or design driven is to use the same base materials while varying the design a bit. This would allow us to compare the results directly and see if our variations have an influence on the outcome. Now that we have the general idea for our test, let's add in a few more specifics. While I don't have a bridge lying around to test, and you probably don't either, we can use some smaller items for experiment. I have here two foam blocks, two wooden skewers of different sizes, two batteries, and a roll of tape. Put together, they form our two test specimens. One end of each skewer is pushed into the foam block, with the other end supporting a battery tape to it. The result is two systems that have the same mass, but slightly different designs. Now let's get to testing. To test our hypothesis, we'll need to induce resonance. By shaking the foam block back and forth, we'll try to excite the resonance frequency. Let's first take a look at the design with a shorter skewer. When we start shaking the block, the battery and the skewer mostly move along with the same motion of the block. And the amplitude certainly isn't increasing. It doesn't appear to be resonating at all. Well, we hypothesize that design doesn't factor into resonance, so we would expect the long skewer design to behave similarly. But wait a second, it's not acting in the same way. When we give this one a shake, the battery almost immediately starts to oscillate at higher amplitudes. This is definitely a distinct difference from the other test we did. So now what can we learn from our experiment? Based on our hypothesis, our oscillating structures should have behaved in the same way, but they didn't. What we saw was that there was a clear difference between the short skewer setup and the long skewer setup. Therefore, we can say that our hypothesis has been disproved, and that the resonance is indeed influenced by structural design. But what does this mean in the greater engineering sense? If we go back to the Tacoma-Nara's bridge, this conclusion makes sense because the bridge wasn't constructed out of different materials than other bridges at the time. The key difference was how the bridge was built and what design decisions were made. Mass, material stiffness, and geometry all play a significant role in structural resonance. And this principle of resonance isn't only applicable to big structures like bridges, but is also used by engineers to drive the design of structures which are joined together and can independently oscillate, such as a spacecraft launch vehicle. Maybe one day you'll look at designing a structure and use resonance frequency to ensure it will operate safely.