| MadSci Network: Engineering |
Hi Jordan. You pop an interesting question! I will do my best to provide a few answers, but you are going to do some of the work with me. When you are done, you may want to turn it into a riddle ... it will be a good one!
First things first ... put on your detective hat. This is a mystery and we are going to solve it together.
You noticed that we could hold the cork or get something to hold it for us. That's a good observation. You also noticed that the cork will eventually hit the cup even if we place it in the middle. The cork moves around in the water and it is just a matter of time before it hits the cup. If you knock the cup, there will be a lot of waves and the cork will hit the side. Game over.
1. Be very very still. Unfortunately, the cork will still hit the side eventually, even if there are no waves. The problem is that the water pushes the cork and then doesn't offer much resistance. A cork moves around in water quite easily -- this is the main problem. I have another idea, but if you really don't want to use any props at all then it will only work in the winter.
2. What exactly do you mean by the edge of the cup? Put your finger on the edge of the cup and trace the edge all the way around. Can you change the amount of water so that the cork can not reach the edge?
3. Put the cup over the sink (you will see why in a minute.) Put your hand over the opening. Tip the cup on its side (see!)
4. Have you ever experimented with an up-side-down cup in the bathtub? I am sure this could lead to another answer, but I can't figure out exactly how ... can you?
5. In answer number 1, we discovered that water may be solid as well as liquid. Can you think of another form that water can take?
I can think of two more, but that would spoil all the detective fun.
This was a great exercise (for both of us) and I hope you continue in your detective ways. Perhaps I will call you if I am stuck on a great mystery of my own.
Here is my favourite riddle. How can you get an egg to stand on its end on a flat table. No props.
Chris Neale.
neale@innocent.com
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Admin note:
Susan Rollinson adds the following:
Great question! A good one for experimentation!
If you float a cork in a cup of water, eventually the cork drifts to the side. As soon as it gets close, it is sucked to the side as if attracted to a magnet.
The surface tension of water causes it to stick to the side as the water molecules would rather interact with each other rather than the side of the cup or the cork. The water is literally pushing the cork out of its way.
So what could disrupt or eliminate the surface tension? We (the whole
family got into the experiments) tried:
dissolving salt in the water
dissolving sugar in the water
adding dish detergent to the water (from a little to a lot!)
adding a layer of vegetable oil over the water
None of these made a difference.
However, try filling the cup with water until the water bulges over the top. The surface tension keeps the water from spilling. Now there is no side above the water to attract the cork, and when it gets to the side of the cup, it just bounces off. I wish I had the facilities to make a QuickTime video of the difference, because it is quite dramatic.
Another solution was to add weight to the cork so it floated just below the surface of the water. (We taped two pennies to the cork.) Again, since there were no surface effects, the cork just bounced off the sides of the cup.
See also:
Why to Cheerios stick together in milk?
Why do boats float and silver sink?
Behavior of pepper floating on water
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