You and Your Students!
Vicki Cobb, Education World Science Editor
Discover why we don’t feel the earth move.
Earth Science, Astronomy, Physical Science
Setting the Scene (Background)
Science is all about questions. One pet peeve I have with all the testing that's going on today is that everybody seems so oriented toward answers, right answers at that. What happens when you answer questions? You shut off the inquiry, that's what! People are uncomfortable with not knowing. But science would not exist if scientists were not fascinated with mystery -- if they were not able to dwell on a question, play with it, and risk being wrong. In this lesson, I want you to give kids a question to think about overnight. Don't give them an answer! Let them be wrong in their answers. Let them have some time to think and discuss before you give them any explanation. Try it. See if it doesn't get them excited!
Here's the question I referred to above. Give this question to your students to ponder overnight:
If the earth is spinning like a top, why don't we feel it move?
OK, if you want to give your students one hint, here's a related question:
If the earth is spinning, why isn't everything that's loose thrown off into space?
Have your students had at least an "overnight" to ponder the question you posed? If so, now you can begin a discussion that will lead them to the best answer scientists have to that question:
To start the wheels turning, ask your students How do you know when you are moving?
We don't feel the earth move because it is spinning at a steady rate; the "ride" is a smooth one. We feel motion only when there is a change in the way we are moving. And a change in motion occurs only when an outside force acts on us. Most of the time we are moving in a car or on an elevator or in an airplane, the ride is not perfectly smooth -- there are bumps and jerks that tell us we're moving. Without those bumps and jerks, though, being in motion would feel no different from being at rest. (Of course, if students look out the window of an airplane and see the ground moving below, they will also know they are moving.)
The reason that loose things are not hurled into space is because everything on the earth's surface is moving with the earth.
Here's something your students do to check that out next time they are riding in a car: Have the driver go at a steady speed. Drop a pencil on the floor of the car while you are moving. Later, when the car is stopped, drop the pencil exactly the same way. Both times it will land on the same spot. The motion of the car does not change the fall of the pencil, as long as that motion is steady. Only a sudden change in the car's motions while you are dropping the pencil will change where the pencil ends up.
When you are sitting in a moving car, you have the same forward movement as the car. But if the car suddenly crashes to a stop, you keep the forward motion and can be thrown through the windshield; hence the reason for seatbelts, car seats, and air bags.
Behind the Scenes
After thinking about the question overnight, some of your students might have concluded that the reason we don't feel the earth move is because it is moving too slowly. The next logical step would be to figure out how fast the earth is moving as it spins. There's an easy solution to that question. The circumference of the earth at the equator is about 24,000 miles. The earth makes a complete rotation on its axis every 24 hours. So the speed of rotation at the equator is about 1000 mph. The next question is: At our latitude, are we moving faster or more slowly than at the equator? Have students look at a spinning top to figure out the answer. Where is the rotation close to zero? (At the poles.)
For more questions to dwell on, read Why Doesn't the Earth Fall Up? And Other Not Such Dumb Questions About Motion by Vicki Cobb, illustrated by Ted Enik.
Article By Vicki Cobb
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