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Make a diver that will rise and fall at your command, if you know the science.

This week's experiment comes from a recent question, wanting to know whether gravity is a law or a theory. That question brings up so many more questions that I thought it would be fun to explore. To try this, you will need:

- an object to drop.

OK, pick an object that will not break, dent the floor, cause a mess, or get either of us in trouble. Hold it out in front of you and release it. What happens? It falls, of course. The gravitational attraction between the Earth and the object pulls it towards the ground. But, when we do this experiment, should we be talking about the Law of Gravity or the Theory of Gravity?

Actually, we should be talking about both. To understand why, we need to understand the scientific mean of the words "law" and "theory."

In the language of science, the word "law" describes an analytic statement. It gives us a formula that tells us what things will do. For example, Newton's Law of Universal Gravitation tells us that "Every point mass attracts every single point mass by a force pointing along the line intersecting both points. The force is directly proportional to the product of the two masses and inversely proportional to the square of the distance between the point masses." That formula will let us calculate the gravitational pull between the Earth and the object you dropped, between the Sun and Mars, or between me and a bowl of ice cream.

We can use Newton's Law of Universal Gravitation to calculate how strong the gravitational pull is between the Earth and the object you dropped, which would let us calculate its acceleration as it falls, how long it will take to hit the ground, how fast it would be going at impact, how much energy it will take to pick it up again, etc.

While the law lets us calculate quite a bit about what happens, notice that it does not tell us anything about why it happens. That is what theories are for. In the language of science, the word "theory" is used to describe an explanation of why and how things happen. For gravity, we use Einstein's Theory of General Relativity to explain why things fall.

A theory starts as a hypothesis, an untested idea about why something happens. For example, I might propose a hypothesis that the object that you released fell because it was pulled by the Earth's magnetic field. Once we started testing, it would not take long to find out that my hypothesis was not supported by the evidence. Non-magnetic objects fall at the same rate as magnetic objects. Because it was not supported by the evidence, my hypothesis does not gain the status of being a theory. To become a scientific theory, an idea must be thoroughly tested, and must be an accurate and predictive description of the natural world.

While laws rarely change, theories change frequently as new evidence is discovered. Instead of being discarded due to new evidence, theories are often revised to include the new evidence in their explanation. The Theory of General Relativity is has adapted as new technologies and new evidence have expanded our view of the universe.

So when we are scientifically discussing gravity, we can talk about the law that describes the attraction between two objects, and we can also talk about the theory that describes why the objects attract each other.

Have a wonder-filled week.

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Hot and Cold Water

Why would an ice cube cause the water in a glass to mix faster?

Last week, I left you with a question about the behavior of a stream of flowing water. I love things like this, because they are things that we see every day, and never really stop to wonder about. Once you do start to look and think, you see wonders that you never noticed before.

We were looking at a stream of water from the faucet, and noticing that as the stream fell, it got thinner and thinner. Now, why would it get thinner?

To understand that, we need to think a bit about how things fall. If you hold a ball out in front of you and drop it, it falls downwards. In the short drop to the floor, you may not notice anything else, but if the drop is longer, you will notice that as the ball falls, it is speeding up, falling faster and faster. That is because gravity continues to pull on it, so it accelerates. The acceleration of gravity is 9.8 meters per second per second. No, that second "per second" is not a typo.

What that tells us is that if you drop the ball, after one second, be falling at 9.8 meters (32 feet) per second. During the next second, it would have been accelerated by the pull of gravity to19.6 meters (9.8 + 9.8) per second. In the third second of its fall, it will be falling at 29.4 meters (9.8 + 9.8 + 9.8) per second.

The process continues, with the ball falling faster and faster, until the resistance of the air balances the acceleration of gravity. At that point, the object won't fall any faster. That is called the terminal velocity. Terminal velocity keeps raindrops from punching holes in the roof, and keeps pennies dropped from sky scrapers from making holes in the sidewalk.

So what does all that have to do with our water? Well, as the water falls, it is speeding up too. That means that the bottom part of the stream is falling faster than the top part. If we were looking at a stream of sand grains, maybe the sand falling in an hourglass, we would see that the grains of sand were very close together as they began their fall, and that they spread farther apart as they fell.

Then why doesn't the water do that? Well, water molecules are very sticky. They stick to each other very strongly, which is what causes surface tension. That keeps them from spreading out, so instead, the stream is stretched and pulled into a narrower stream, sort of like pulling on a piece of chewing gum.

As the stream of water falls faster and faster, the stream is stretched thinner and thinner. Eventually, the stretching reaches the point where it overcomes the stickiness of water, causing the stream to break up into separate water drops. You can see that by barely turning on the faucet, to create a very thin stream of water. Near the bottom of the stream, you will see it starting to break up into drops.

Sometimes it helps if you slow things down a bit, to make them easier to see. Galileo did that with the acceleration of gravity, studying balls rolling down in inclined plane instead of falling. We can do the same thing, with something thick, like a stream of chocolate syrup, falling onto a bowl of ice cream. Does it behave in the same way as a falling stream of water? You will have to try it yourself to find out. Now that is some fun homework!

Have a wonder-filled week.

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Pendulum Perils

Are you brave enough to try this?

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Soil Types

Exploring the science of dirt.

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Sand Angles

Learn structural engineering by making piles of candy.

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Turning Ice

How can watching ice melt be interesting? Try it and see.