I just saw this video of an anti-gravity fountain on youtube, but I can't understand how you can reverse the flow of water.
I know you can stop the flow of water by using something called a stroboscope, but how do you reverse it?
$\begingroup$
$\endgroup$
3
-
7$\begingroup$ I know you can stop the flow of water by using something called a stroboscope, but how do you reverse it? You can't "stop the flow of water by using something called a stroboscope", the strobe just makes it look like that. Adjust the frequency and the drops appear to levitate. See also car alloys rolling in reverse. $\endgroup$– GertCommented May 2, 2019 at 16:28
-
3$\begingroup$ Hi and welcome to physics.SE! It's currently unclear what exactly this question is asking without clicking on the link you provided. To make questions more accessible and guard against link rot, please include all relevant information, such as a description of the phenomenon observed in that video, in your question. $\endgroup$– ACuriousMind ♦Commented May 2, 2019 at 17:20
-
1$\begingroup$ @Gert Yes but how can you make it seem like it goes backwards ? $\endgroup$– ng.newbieCommented May 3, 2019 at 18:13
Add a comment
|
3 Answers
$\begingroup$
$\endgroup$
It’s an optic illusion. A strobe light is used to illuminate the water droplets and the trick is very simple. If the light is strobing at the same frequency as the water droplets, the drops seem stationary because every time the strobe flashes it is illuminating the next water drop at the same location (in the drop) as the previous one. So changing the frequency to slightly slower will make the drops appear to fall in slow motion as the next drop gets illuminated (or arrived by the light) at a slightly lower position as the previous drop. And finally, if the strobe frequency is slightly higher it will ilumina-te the next droplet at a slightly higher position, thus giving the illusion that the drops are moving backwards. It’s just our eyes playing tricks.
$\begingroup$
$\endgroup$
You already have a very nice answer, just let me point out that you probably have already seen this effect at work: sometimes very fast-moving wheels seem to move in reverse.
The flow in the fountain is not reversed, the droplets are falling. However, if you create a "periodic" system of droplets (in fact there are several droplets that are almost equally spaced), you can use the illusion given by the " wagon wheel effect ". After all, the periodic system of droplets is just an "unrolled wheel" and the effect is due to a sampling error of a periodic signal (every time the stroboscope light switches on your eyes take a "snapshot" of the droplet's actual configuration).
Here you can play with a periodic signal and see what happens when you modify the sampling rate.
Note#1: also the neon lamps have a discharge frequency of ~50 Hz, so they can create a stroboscope effect. Any machinery rotating at multiples of this frequency may appear to not be turning and this can give rise to safety problems in factories.
Note #2: if you really want to levitate water droplets, you may use something like an acoustic levitator.
$\begingroup$
$\endgroup$
Let the water droplet fall at a constant frequency. When we cover our eyes with a large shutter, we adjust the opening and closing frequency of the shutter. When the frequency matches the frequency of the water droplet falling, we will see that the water droplet is stationary, and when the frequency is relatively high, we will see that the water droplet is moving upwards. When the frequency is relatively low, water droplets move downwards.
In addition, with such a shutter in front of the eyes, we can also see the slow flight of the bird and the slow rotation of the wheel hub. Even static flying birds and stationary rotating wheels can be seen.
