So recently, I wrote a blogpost about using video analysis to analyze the motion of a falling object. As you can probably tell, I’m fascinated by falling objects and not only do I love to analyze the movements but I also like to try new things so I prepared an experiment that is so well known and amazes me every time I see it. I hope it will fascinate you as well.
This experiment is so powerful and so easily set up, how can you not love it from the first moment. All it took for me were two objects (are they the same? Do they differ from each other?) and two pipes. Obviously, the pipes are different to each other: the darker one is made out of plastic polymer, the second one is made of iron. We will come back to the question about the objects later. So let the experiment begin.
I’ll start with the polymer pipe first, letting both objects fall through it after each other. I’ll admit, I would rather see them fall and watch their motion in slow motion and maybe video analysis. But I promise, it’ll get really great.
Alright, that was a wee fast. I hate when objects don’t slow down. Thanks, gravity! How am I supposed to tell the difference between the objects? But since they were both so fast, we are safe so say they behave the same. Still, let’s take a look at the slow-motion falling part.
Still, you probably ask yourself why would I even bother to ask if the objects are the same if they really were. Wouldn’t that be boooooring? You’re right. And the following drop through the iron pipe will show you just how right you are.
I fastened up the video a bit, since, honestly, there is nothing much to see. The object falls through the pipe, just as fast as it did through the polymer pipe. Let’s take a look at the second object.
And there you have it. They’re not the same and you’re probably already guessing what is “wrong” with the second object. It’s a neodymium magnet and the reason why it’s falling so slowly can be found in Lenz’s law! The falling magnet induces a current in the iron pipe, which in turn produces a magnetic field. But the direction of this induced current then opposed the change in the magnet’s field, resulting in the magnet being repelled. This leads to the magnet falling more slowly.
There is some serious physics going on there and students can explore Lenz’s law in a somewhat magical and playful way with the certain wow-factor. If you’re interested in the unshaved and utterly brilliant physics behind it, you can find an article about the magnet falling inside a conductive pipe right here.
I still had the pipes, the magnet and the small object (which is acutally also made out of iron, just like the tube) from a demonstration of the experiment when I wanted to show how easily it can be set up. But there are several other demonstrations on the internet that also show how the magnets fall through the tube and this one is probably my favorite one: