Why are the concepts of mass and weight so confusing?

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This week's experiment comes from an email I got from a student. (Thanks Darius!) He wanted to know what causes your knuckles to pop. To try this, you will need:

- your hands

First, not everyone's knuckles will pop, and some pop more than others. Other joints in your body may also pop, some for the same reason as your knuckles, and some for other reasons.

There are different techniques for popping your knuckles, but they all work in basically the same way, by forcing the two bones of the joint to move farther apart. Probably the simplest way to do this is to interlace the fingers of your hands. Then turn your hands so that your palms are away from your body, and gently bend your fingers backwards. THIS SHOULD NOT HURT. DO NOT FORCE YOUR JOINT TO THE POINT WHERE IT IS PAINFUL. As the pressure builds, you should hear a pop from one or more of your knuckles.

If you search the internet for the answer to this question, you will find all sorts of answers, ranging from good explanations to wild guesses and pure fiction. The most comprehensive answer that I found was from "A bioengineering study of cavitation in the metacarpophalangeal joint" by
A. Unsworth, D. Dowson, AND V. Wright, from the Bioengineering Group for the study ofHuman Joints, the University of Leeds. If you want to read the article, you can find it here: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1005793/

If you don't want to wade through the article, the sound is caused by a process called cavitation. OK, so what is cavitation?

Lets think for a moment about the properties of liquids. Liquids take on the shape of their container, but they maintain their volume. You can't squeeze water into a smaller space, or stretch it to fill a larger space. If you try to stretch water into a larger space, at first, nothing seems to happen, but you are reducing the pressure of the liquid. When the pull gets strong enough, and the liquid pressure is low enough, some of the liquid changes to a gas, forming a bubble. Unlike the liquid, the gas in the bubble can be stretched into a larger space. That reduces the pull on the liquid, which raises its pressure. At that point, the gas in the bubble almost instantly changes back to a liquid, collapsing the bubble.

The same thing happens when you pop your knuckles. As you apply pressure to your knuckle joint, it forces the ends of the bones apart. Surrounding the joint is a liquid called synovial fluid. Moving the bones apart pulls on the fluid. If it was a gas, it would expand to fill the space as the bones separate. Since it is a fluid, it stays the same size, and its fluid pressure decreases. As you pull harder, the fluid's pressure gets lower and lower, until it reaches the point where the pressure is low enough to let some of the liquid change to a gas, forming a bubble. The bubble expands in response to the pull, which lowers the stress on the fluid. That lets the fluid's pressure go back to normal, which lets the gas change back into a liquid, and the bubble collapses. That collapse produces a loud sound.

Cavitation can be seen in other situations where liquids are subjected to very low pressures. Submarines have to be careful about cavitation from their propellers, as the sound can give away their position. An animal called the Pistol Shrimp snaps its claw fast enough to cause cavitation, producing a sound that is loud enough to stun or kill nearby fish.

Another question that frequently pops up with this subject is why you can usually only pop a joint once, and then you have to wait 20 to 30 minutes before it will pop again. When the low pressure causes the bubble of water vapor, it also causes some of the dissolved gases in the fluid to form bubbles. These bubbles, made up mostly of carbon dioxide, do not instantly collapse, remaining until the gases are reabsorbed back into the fluid. This usually takes 20 to 30 minutes. If you try cracking the joint again before those bubbles are reabsorbed, the bubbles of gas expand, preventing the pressure buildup that causes cavitation.

The other question that comes up frequently is whether knuckle cracking causes arthritis or other damage. Not many studies have been done, but those that have been done do not show any link between knuckle popping and arthritis.

Have a wonder-filled week.

This experiment comes from a question sent in by a list member who wanted to know how a drinking straw works. At first this seems to be a very simple thing, but like most very simple things, the more you try to explain it, the more complicated it gets. To explore this subject, you will need:

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This week's experiment comes from a question sent to me by 10 year old Will Boyd. His question was, "If I were standing directly on the South Pole and I was holding a compass, where would the needle be pointing?"

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Open any book or web page that talks about magnets, and you will probably see a drawing that shows magnetic lines of force that extend from one end of the magnet to the other.

Are there really lines of magnetic force as they show in the drawings? Well, lets find out. To try this, you will need:

- a file
- iron nails
- a small, plastic bottle cap
- clear, dish washing soap
- a strong magnet
- metal paper clips

If you tried the experiment from the Science of Credit Cards video, don't throw away the iron filings. If you did not, then you can use the file and iron nails to make about 1/4 teaspoon of iron filings.

Fill the plastic bottle cap about 3/4 with the dish washing soap.

Then stir the iron filings into the soap, until they are evenly distributed.

Place the bottle cap on top of a strong magnet. Watch the iron filings carefully as you put the magnet in place. You should notice that the filings are attracted to the magnet.

Looking closer, the magnetic filings stick together, forming lines that look very much like the drawing in the book. So there really are lines of magnetic force, right? No.

Wait a minute! What do you mean by "No"? I can see the lines, right there in front of me!

Well, there are lines of iron filings, but there are no lines of magnetic force. The magnetic field that extends around the magnet does connect one end to the other, but it is spread evenly, not in specific lines of force. Then why do the filings line up like that? To find out, lets look at some paper clips.

Pour a pile of paper clips on the table, and put the magnet into the pile. When you lift the magnet, the paper clips stick together, forming magnetic chains.

Each of the paper clips in the chain now has a north and south magnetic pole. In the photo, the paper clips are sticking to the north pole of the magnet, so on the first paper clip, the end that is touching the magnet will be a south magnetic pole. The other end will become a north magnetic pole, attracting the next paper clip in the chain. That paper clip will also develop north and south magnetic poles, and will attract the next paper clip in the chain.

The same thing happens to the iron filings. They attract each other, end to end, to form chains, just as the paper clips did. That is what forms the lines you see, not invisible lines of magnetic force. How do we know?

Remove the magnet, and stir the soap again. Put the magnet back, and you will see the lines form again, but if you look closely, you will notice that they are not in exactly the same place. If there really were actual lines of force, the filings would line up in the same place each time. Since the magnetic field surrounds the magnet evenly, instead of in lines, the locations of the chains of iron filings are random each time. You always get the chains, but they are always in different places.

The lines in the drawings work well for explaining magnetic fields, and the chains of iron filings reinforce that idea, but when you observe carefully, there are no magnetic lines of force.

This week's experiment comes from a question that was sent to me by Hashi, one of the members of the Experiment of the Week list. She noticed that smells were stronger while taking a shower and asked why. To investigate, you will need:

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A few weeks ago, we looked at a classic experiment of putting a lit candle under a glass. We saw that water was drawn into the glass, not by the oxygen being burned up, but by the cooling of the air in the glass after the candle went out. This experiment will allow us to do what many people thought the first experiment was doing, measure the amount of oxygen in the air.

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For this week's experiment, I wanted something that related to hurricanes. I settled for one that is based on fast moving air and differences in air pressure. You will need:

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This experiment comes from a series of news spots that I saw regarding the spring equinox. The equinox occurs on the day when winter changes to spring and when summer changes to fall. There is a persistent story that during the equinox is the only time that you can stand an egg on its end. I did a news search and found hundreds of articles from newspapers and television with stories about this. In reality, the equinox has nothing to do with it and your chances of standing an egg on its end are the same all year round.

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We are used to thinking of things as falling into the basic groups of solids, liquids, and gases. (In another experiment we will discuss a fourth state of matter, plasma.) In this experiment, we will examine a substance that sometimes acts like a solid and at other times acts like a liquid.

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Why does making something wet cause it to look darker?

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Sometimes the right answer is not the only answer.

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It is amazing how many books get this one wrong!

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How fast can you throw a piece of paper?

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