When paper is cut to a small width, it curls up like this. Why does this happen?
[Physics] Why does finely cut paper curl
home-experimentmaterial-science
Related Solutions
Instead of a circular hole, let's think of a square hole. You can get a square hole two ways, you can cut it out of a complete sheet, or you can get one by cutting a sheet into 9 little squares and throwing away the center one. Since the 8 outer squares all get bigger when heat it, the inner square (the hole) also has to get bigger:
Same thing happens with a round hole.
This is confusing to people because the primary experience they have with stuff getting larger when heated is by cooking. If you leave a hole in the middle of a cookie and cook it, yes, the cookie gets bigger and the hole gets smaller. But the reason for this is that the cookie isn't so solid. It's more like a liquid, it's deforming. And as Ilmari Karonen points out, the cookie sheet isn't expanding much so there are frictional forces at work.
What you are seeing is the result of pressure waves moving upstream against the flow of the water.
Pressure waves in a fluid move at the speed of sound (because sound is a pressure wave) and in water this is around 1.5 km/sec. So while the water itself is moving downwards the pressure change caused by hitting an obstacle can propagate upstream as long as the speed of the water flow is less than 1.5 km/sec. If you inserted a pressure gauge in the water where it leaves the tap you would see the pressure increase when you put the spoon in the stream of water. This increase in the pressure in effect inflates the column of water, and this gives the effect you're seeing in the video.
To actually calculate the effect and predict its magnitude would be a formidable problem because even simple problems in fluid dynamics can be computationally intractable. The usual tactic is to give up and reach for a finite element analysis program and a large computer. I mention this to give myself an excuse for not explaining why you get the pulsing effect in your second video. It too will be due to pressure waves travelling upstream, but exactly why you get the oscillation I don't know.
Best Answer
When it happens for a paper you hold above the floor, one of the ways to deal with the forces and deformation. If you are holding the paper at an angle, not vertically, paper closer to the point where you hols it is all that does not let the farther parts just hang down. Now, paper is not really good at shear deformation, but if it so happens that a stripe bends left or right, it keeps doing so, because slightly bending is just an easier deformation and nothing prevents it from doing so.
However, I think you are talking of stronger curls, and that probably has to do with the way you cut it up, bending it as you go through. I just sliced off a thin stripe of paper with boxcutter — almost no deformation occured.
Note also that if we are talking about really large sheets of paper, the paper during its creation is pressed several between cylinders and then rolled onto reels. So small (or not so small) curling is quite typical.