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Archive for the ‘hydraulics’ Category

As you might have noticed, some objects float and some others don’t. Here below I have a rendition of a boat and a cube of ice floating.

 

Various floating objects

Various floating objects

 

 

Today, I will go on a bit about flotation. As a matter of fact, some of you might remember a puzzle with an egg I wrote down a while ago. Of course, most of you have probably heard of Archimedes Principle as describing flotation, so I will explain some aspects of how that principle comes about.

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Candle in an open container spinning in a centrifuge

Candle in an open container spinning in a centrifuge

So here we have a physics puzzle to tease you a little bit. The idea is simple. You put a candle in an open container (let us say a glass, like in the restaurants). You attach the container to a centrifuge, with a long arm, and you make the centrifuge spin at high speeds.

 

The big question is: what happens?

 

 

This is sometimes the flavor of big physics questions. One puts a couple of ingredients together, sometimes in a lab, and sometimes as a theoretical exercise, and one is supposed to come up with a detailed description of what happens. In this problem there are many ingredients, and it is not obvious what is important and what is not. This is what makes these problems fun. This problem was suggested to me by Nick Warner a couple of weeks ago. I thought it would be good to release it with some picture.

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You might have seen them in your shower, kitchen refrigerator or walls, or even you might have seen them hanging from the glass panel in the rear of a car you are following. They are suction cups, the product that really sucks. Erm, maybe that is not the best description of these objects, since they are very cool and useful and they don’t suck. Well, they do, but in the sense of suction, not in the other colloquial sense.

 

A cross section of a suction cup attached to a wall

A cross section of a suction cup attached to a wall

 So, how do these little suckers work? That’s the post for today. I have not explained any device recently, so I thought these would be a nice addition to the everyday physics series.

 

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So we were welcoming the new students to the UCSB physics department last week. I believe that it has become a tradition to make ice cream with liquid nitrogen in the physics department. It is very yummy and it’s so cool. Moreover, it takes just a few minutes. Above is a movie on how not to do it. If you recall the pressure cooker explanation, I warned about the dangers of evaporating
fluids in a closed container and the ensuing pressure buildup. In this recipe, the heat is provided by the ice cream ingredients, and the nitrogen is the liquid that you are boiling. In the movie you get a graphic explosion due to excessive evaporation of nitrogen and it is so very funny. For the future, if you ever try this recipe, remember to do it in an open container! You should also learn to use wooden spoons and do it by hand. It is much more fun than a blender and it makes much less of a mess.

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I thought I would give you a physics puzzle to solve for a change. This puzzle was posed to me this week right before lunch and we had a fun discussion with my colleagues about it. The great thing about the puzzle, is that you can check for the solution in your kitchen if you have a machine to measure weights. Many cooks do, so you can become an experimental physicist for a day if you want to. The puzzle involves typical things that you can find in your home, like water, and a container for it. It also involves lead (in the original puzzle), which is substituted by a blue egg in the picture. If you have never seen blue eggs, go to a traditional chinese food store near your place and ask for duck eggs. So now you have all the ingredients.

A blue (lead) egg suspended in water hangs by a string


So the idea is that you suspend the egg from a string (you can use superglue), and you lower it slowly into a container of water, but the string supports the weight of the egg. You can hang the string from your favorite support, like your finger. The water is seating atop an instrument that measures its weight. The question is: if you lower the egg in the water, will the weight marked in the register increase, stay the same, or decrease. There is more than one way to think about the puzzle, but in the end you should always arrive at the same answer. The really important question is if the weight on the measuring device changes, by how much does it change?

Incidentally, this reminds me of a time when I was in graduate school when lead was found in the water pipes of the university I was in, many many years ago. The water fountains in the building were closed with yellow warning tape, and right next to them a big water jug dispenser was placed so that people could still get water, but not from the tap. Someone with a good sense of humor (I imagine it was a physics graduate student) put signs on the water fountain from the pipes and the bottled water. The signs said “Leaded” and “Unleaded”.

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Every theorist (read that theoretical physicist) knows that without coffee there is no physics. Well, kind of. Every time I visit another physics department, after I am shown my office and how to get my computer connected to the web (also indispensable for physics), I am taken with reverence to the local coffee machine and I am explained with great detail on how it is supposed to be operated. Coffee seems to be the lubricant of good ideas.

Now you can probably believe, without reason, that coffee is actually good in physics departments. Well, in my limited experience, it varies a lot. From great coffee to lousy coffee. But the caffeine is always there. What is sure is that physicists drink coffee regularly and that is a fact. Now, being as I am, born from a country where coffee is a main export item, I can be very picky as to how good coffee is supposed to taste. Also as to how it is supposed to be prepared. I can go at length about it. So I thought, how can I talk about coffee and physics at the same time?

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