# 2nd harmonic oscillation of a rope full with balloons — and announcing a summer break

On one of my very last days in my old job, I walked across campus and noticed this:

I had to, of course, alert all my nearby friends! Especially since, on this engineering campus, apparently nobody else stopped to take a quick video. Seriously, what’s wrong with the world?

I obviously had to come back at different wind conditions to get a better dataset, which I proudly present in the movie below:

It felt a bit like the university was waving me goodbye, and a part of my life was ending. Which is not a bad thing necessarily, but before I plunge head-first into my upcoming research cruise and then the new job and all the other small or not-so-small commitments that I’ve made, I am going to take it slow(er), take a break and reevaluate my priorities. And for that I will be taking a summer break from regular blogging, too, so you might not hear from me for a bit.

I’ve had this song from the 2007 Hitchhikers Guide to the Galaxy movie stuck in my head for days now. So here we go:

So long, and thanks for all the fish!

# Velocity of shallow water waves.

The experiment we run to discuss the velocity of shallow water waves.

In this post, I discussed how it took us several years to modify an experiment to make it both student and teacher-friendly. But what can you actually see in that experiment?

The movies below show the type of standing waves that are excited in the tank. This movie for 24 cm water depth (Ha – this is going to come back and haunt me! I’m not actually sure what the water depth in this experiment is. It looks like this is the case with the highest water level we have run. But if you want to know for sure go ahead, measure the period, calculate the phase velocity (the tank is 175 cm long) and then calculate the water depth. Good practice! ;-))

And then this movie shows the experiment with a lower water level (12 cm? 8? I don’t remember).

It’s interesting to see how much more difficult it is to excite a nice standing wave if you have less water in the tank. Intuitively that makes sense, but does anyone have a good, not-too-theoretical explanation?

# Seesawing of standing waves.

Improving one of the experiments run in the GEOF130 lab.

One experiment that has been run in GEOF130 forever is the “standing wave”, where a wave is excited in a long and narrow tank and then, for different water depths, the period is measured and the velocity calculated in order to compare it to the one calculated from the shallow water wave equation.

Traditionally, the standing wave is excited by lifting one end of the tank, letting the water settle down, and carefully putting the tank back down. This, however, means that someone has to lift a pretty heavy weight. So Pierre and I were quite proud of ourselves when we constructed a pulley system last year and now instead of lifting the weight up, someone could hang on a rope instead.

However, this was still hard work, and even though the picture shows a student doing the lifting, for most lab groups it was actually Pierre who did it.

But then this year, we came up with a much simpler solution and I don’t know how we didn’t see this before now. As Pierre remarked: We talk about seesawing standing waves ALL THE TIME. How did it not occur to us that the simplest setup would be a seesaw? So now we have two wooden blocks underneath the tank, one supporting it in the middle and one underneath the end where the operator is standing. So all that needs to happen now is a slight lift of the tank and then a slight downward push to bring it back in the horizontal.

So much easier!

# Standing waves.

A seesaw to visualize how standing waves move in an enclosed basin.

In enclosed basins, standing waves can occur. In the simplest case, they have a node in the middle and the largest amplitudes at the edges of the basin. The movement of the water’s surface then closely resembles that of a seesaw.

A seesaw. Largest amplitudes at the ends, node in the middle.

Extremely simple but extremely effective visualization!