Month: February 1999

My 5 year old wants to do his kindergarten science project on “why do balls bou…

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My 5 year old wants to do his kindergarten science project on “why do balls bounce?” His hypothesis is that “balls bounce because of the stuff inside.” Can you advise how best to test this hypothesis and explain this concept on a level that a bright, but still only 5 year old, can truly understand? — MS, Bayside, New York

I’d suggest finding a hollow rubber ball with a relatively thin, flexible skin and putting different things inside it. You can just cut a small hole and tape it over after you put in “the stuff.” Compare the ball’s bounciness when it contains air, water, shaving cream, beans, rice, and so on. Just drop it from a consistent height and see how high it rebounds. The ratio of its rebound height to its drop height is a good measure of how well the ball stores energy when it hits the ground and how well it uses that energy to rebound. A ball that bounces to full height is perfect at storing energy while a ball that doesn’t bounce at all is completely terrible at storing energy. You’ll get something in between for most of your attempts—indicating that “the stuff” is OK but not perfect at storing energy during the bounce. The missing energy isn’t destroyed, it’s just turned into thermal energy. The ball gets a tiny bit hotter with every bounce.

You won’t get any important quantitative results from this sort of experiment, but it’ll be fun anyway. I wonder what fillings will make the ball bounce best or worst?

I saw a magic show where they put a needle through a balloon. I tried this and i…

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I saw a magic show where they put a needle through a balloon. I tried this and it worked, but only with latex material balloons. I want to do my science project on this but my teacher said it was not a good idea. I think that it is because it is science, not magic. What do you think? — J, 6th Grade

It is science. The needle is able to enter latex without tearing it because the latex molecules are stretching out of the way of the needle without breaking. Like all polymers (plastics), latex consists of very large molecules. In latex, these molecules are basically long chains of atoms that are permanently linked to one another at various points along their lengths. You can picture a huge pile of spaghetti with each pasta strand representing one latex molecule. Now picture little links connecting pairs of these strands at random, so that when you try to pick up one strand, all the other strands come with it. That’s the way latex looks microscopically. You can’t pull the strands of latex apart because they are all linked together. But you can push a spoon between the strands.

That is what happens when you carefully weave a needle into a latex balloon—the needle separates the polymer strands locally, but doesn’t actually pull them apart or break them. Since breaking the latex molecules will probably cause the balloon to tear and burst, you have to be very patient and use a very sharp needle. I usually oil the needle before I do this and I don’t try to insert the needle in the most highly stressed parts of the balloon. The regions near the tip of the balloon and near where it is filled are the least stressed and thus the easiest to pierce successfully with a needle. A reader has informed me that coating the needle with Vasoline is particularly helpful.

One final note: a reader pointed out that it is also possible to put a needle through a balloon with the help of a small piece of adhesive tape. If you put the tape on a patch of the inflated balloon, it will prevent the balloon from ripping when you pierce the balloon right through the tape. This “cheaters” approach is more reliable than trying to thread the needle between the latex molecules, but it’s less satisfying as well. But it does point out the fact that a balloon bursts because of tearing and that if you prevent the balloon from tearing, you can pierce it as much as you like.

How does a dehumidifier work? – S, Hong Kong

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How does a dehumidifier work? – S, Hong Kong

A dehumidifier makes use of the fact that water tends to be individual gas molecules in the air at higher temperatures but condensed liquid molecules on surfaces at lower temperatures. At its heart, a dehumidifier is basically a heat pump, one that transfers heat from one surface to another. Its components are almost identical to those in an air conditioner or refrigerator: a compressor, a condenser, and an evaporator. The evaporator acts as the cold surface, the source of heat, and the condenser acts as the hot surface, the destination for that heat.

When the unit is operating and pumping heat, the evaporator becomes cold and the condenser becomes hot. A fan blows warm, moist air from the room through the evaporator coils and that air’s temperature drops. This temperature drop changes the behavior of water molecules in the air. When the air and its surroundings were warm, any water molecule that accidentally bumped into a surface could easily return to the air. Thus while water molecules were always landing on surfaces or taking off, the balance was in favor of being in the air. But once the air and its surroundings become cold, any water molecules that bump into a surface tend to stay there. Water molecules are still landing on surfaces and taking off, but the balance is in favor of staying on the surface as either liquid water or solid ice. That’s why dew or frost form when warm moist air encounters cold ground. In the dehumidifier, much of the air’s water ends up dripping down the coils of the evaporator into a collection basin.

All that remains is for the dehumidifier to rewarm the air. It does this by passing the air through the condenser coils. The thermal energy that was removed from the air by the evaporator is returned to it by the condenser. In fact, the air emerges slightly hotter than before, in part because it now contains all of the energy used to operate the dehumidifier and in part because condensing moisture into water releases energy. So the dehumidifier is using temperature changes to separate water and air.

As part of Math and Science night at her school, my 4th grade daughter recently …

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As part of Math and Science night at her school, my 4th grade daughter recently made ice cream. How did the milk, ice, salt, and mechanical motion work together to make ice cream? — DH

To make good ice cream, you want to freeze the cream in such a way that the water in the cream forms only very tiny ice crystals. That way the ice cream will taste smooth and creamy. The simplest way to achieve this goal is to stir the cream hard while lowering its temperature far enough to freeze the water in it and to make the fat solidify as well. That’s where the ice and salt figure in.

By itself, melting ice has a temperature of 0° C (32° F). When heat flows into ice at that temperature, the ice doesn’t get hotter, it just transforms into water at that same temperature. Separating the water molecules in ice to form liquid water takes energy and so heat must flow into the ice to make it melt.

But if you add salt to the ice, you encourage the melting process so much that the ice begins to use its own internal thermal energy to transform into water. The temperature of the ice drops well below 0° C (32° F) and yet it keeps melting. Eventually, the drop in temperature stops and the ice and salt water reach an equilibrium, but the mixture is then quite cold—perhaps -10° C (14° F) or so. To melt more ice, heat must flow into the mixture. When you place liquid cream nearby, heat begins to flow out of the cream and into the ice and salt water. More ice melts and the liquid cream get colder. Eventually, ice cream starts to form. Stirring keeps the ice crystals small and also ensures that the whole creamy liquid freezes uniformly.