My niece asked me to write about this popular (and fake) trick with a glass of water.
In case you don't want to watch the video, it basically shows a glass of water. The glass full of water is placed upside down on a counter top (ok, this part is actually kind of cool - old, but cool). Next, with a few perfectly practiced magic swirls of the water the glass is removed leaving a glass shaped mass of swirling water. Of course this works because everything spinning is always more stable than non spinning things. Plus, you will probably try this and make a mess of water all over the place. At least it is just water.
You may notice that I went ahead and called this one fake. I don't normally do that (except for those stupid power balance bracelets). So, how can I be so confident in this case? It's simple really. Let me start with an upside down glass of water. Suppose a take a small section of this non-rotating water. If everything is still, that section of water should be at rest. Here is a diagram showing the forces on that water.
This looks crazy - but let me explain. First, let me just look at the vertical forces. Of course there is the gravitational force on this piece of water pulling down. Why doesn't this block of water move? Because of the net force from the other water pushing on it. The water below it pushes up more than the water above it pushes down. Why? Because the pressure increases as you go deeper in the water. Since I have a cubical shaped block of water with the same area on the top and bottom, greater pressure means greater force. This is why things float in water (and it doesn't have to have the same sized areas on the top and bottom).
What about the horizontal forces? Well, since the right side of the water block is underwater, there is a force pushing to the left from the water. The water block is in equilibrium because the side of the glass also pushes to the right with the same force as the water pressure force. Simple, right? Oh, and I picked a section of water on the side so that we could see what happens when the glass is removed.
Ok, now take away the glass with non-spinning water. What changes? Here is another diagram.
The only changes are the absence of the glass and the force from the wall. But now there is only one force pushing horizontally. How can this water be in equilibrium? Oh, it can't. SPLASH.
But really Anakin Skywalker had the solution from the Phantom Menace.
Isn't it obvious? You need to spin the water. Spinning the water will make a centrifugal force pushing the water in. Oh. Wait. That isn't right. If that block of water shown above is moving around a vertical axis it would be accelerating towards the center of the circle that it is moving in. There would need to be even more force pushing to the right instead of no forces. If you prefer to think of this in terms of centrifugal forces, the fake force would be pushing AWAY from the water. Spinning is not the answer.
I guess the reason people like the spinning answer is for two reasons.
- Spinning things are more stable. This is usually true for rigid objects, not fluids.
- Spinning stuff makes a force. Sort of true. If you want to think in the rotating frame, there would be a fake centrifugal force. But this fake force would be pushing AWAY from the circle, not towards. I guess this along the same lines as people thinking that the rotation of the Earth causes the gravitational force (it doesn't).
So, the video is fake. On top of that, there are other hints:
- It would be a remarkable event - but there isn't a tripod?
- Also, not uploaded in high definition? Even my phone does HD video.
- Oh, did they mention is it super hard to do? You will probably try and fail - not because it is fake, it's just hard.
I would also look at an analysis of the motion of the camera, but the interesting parts just weren't long enough. Actually, this is not true. I went back and looked at the video BEFORE the trick. Here is a plot of the motion of the background during that time.
In case you can't tell, this is not even close to the background motion from someone holding a real camera (from a previous fake video analysis)
So, what the video creators do is to first use a tripod. Using a tripod makes it easier to add in effects later since the background stays still. Of course it is a big hint that the thing is fake if you use a tripod so they add in this "fake shake" to make it seem more believable.
But here is the part I don't get. Why would you add this fake video shake to the beginning of the movie where you aren't even adding special effects? Couldn't you just really hold a camera and create real camera shake?
