system
Given the following post, respond as if you're commenting on Reddit:
human
Why do certain objects have 'resonant frequencies'?

I accidentally posted this in AskReddit a few hours ago; hopefully I will get better answers here. 

So, I realize that small particles like electrons have wavelengths associated with them (deBroglie wavelengths). What I don't understand is how certain larger objects like glasses, bridges and buildings can have 'frequencies' associated with them. Does this have anything to do with deBroglie wavelengths? What causes these 'resonance disasters' I hear about (like buildings falling over, bridges collapsing, or glasses bursting)?

Thanks everyone!
gpt
This answer might be unsatisfying because it involves math.

Anything that restores itself when it's been deformed we can call elastic. The simplest example is a spring, but it could be like the stresses on a bridge or an airplane wing or something. When an elastic object is deformed, it moves back to its equilibrium position. But because objects in motion tend to remain in motion, it keeps going past equilibrium: tension becomes compression. Then, the reverse happens: it gets pulled back to its equilibrium the other way, and keeps going, etc. Through this process it oscillates.

Now the math part: the total force acting on the object is equal to its mass (m) times its acceleration (a), and is also equal to the elastic restoring force (k), which is proportional to distance (x). You can set up an equation to see that a+kx=0. The solution to this equation is a sine or a cosine wave, with a frequency equal to the square root of k/m. That is the frequency. So anything with a restoring force will oscillate at that frequency.