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This is Members Only content.

Become a Member Today at the Introductory Price of
Only $20 for an Entire Year, and get full access to this site.

Use a globe of the Earth to learn about seasons, day and night, and the scale of the solar system.

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.

This is Members Only content.

Become a Member Today at the Introductory Price of
Only $20 for an Entire Year, and get full access to this site.

What does it mean if something is radioactive?

I am sure that many of you are wondering about the cake and Jello that I mentioned last week. I intended to write more about earthquakes, but the e-mail very quickly got too long. I decided to save half for this week, but forgot to change the materials list. This week, you will need:

a large plate
cake
Jello or something similar

Anytime there is an earthquake, the news media makes a big point of talking about the Richter Scale, using it to indicate how bad the earthquake was. As we will see, the Richter Scale does not tell us nearly as much about surface effects as people think it does. Instead, it tells us how much energy was involved in the quake. That may seem like a small point until you think about it for a minute. An earthquake from a fault that is deep in the Earth may not cause as much damage as an earthquake from a fault at the surface, even if they release the same amount of energy.

Even at the same depth, there are other things that can cause major differences in the surface effects of an earthquake. Place a large, dinner plate on a flat surface. Cut a square of cake and place it on one side of the plate. Cut a square of Jello the same size and place it on the other side of the plate. Imagine that the plate is part of the crust of the Earth. (Insert your own joke about plate tectonics here.) Then imagine a tiny city built on top of each of the two squares. Shake the plate from side to side to simulate an earthquake. Which of the cities do you think would have the most surface effects?

Think of the city on the cake as a city built on solid rock, while the Jello represents a city built on layers of sand, gravel and clay. With the same amount of energy, differences in geology can give you very different results.

Another thing that many people don't realize about the Richter Scale is that it is a logarithmic scale. WAIT! Don't run away! I promise to keep this simple. Because the intensity of earthquakes varies so much, they use a logarithmic scale to make it manageable. It simply means that the vibrations of an earthquake with a 3 rating are ten times stronger than for one with a rating of 2. A 4 is ten times stronger than a 3, and so on. If you look at the total energy, it gets even more impressive. Because the energy radiates in all directions, the energy difference between each number on the scale is more than 30 times greater. That means that while the vibrations of a 4 are ten times stronger than a 3, the total energy released is more than 30 times stronger!

This means that as you go up the scale, the points become much more important. When the news media reported on the recent quake, some said it was a 9, while others said it was an 8.9. That does not sound like much difference until you look at the way the scale works. The difference between 8.9 and 9 is bigger than the difference between a 1 and an 8.

There is a different scale which is much more useful to the average person. The Modified Mercalli Scale is based on surface effects, so it gives you a much clearer idea of the impact of the quake. Instead of a single rating for each earthquake, you get a map with zones to show how much impact it had on each area. You can find a copy of the scale at:

http://earthquake.usgs.gov/learning/topics/mercalli.php

I know that it will take a while to digest all this information, so I suggest you digest the cake and Jello at the same time.

Have a wonder filled week.