Ask the Dreadlocked Science Geek

[dreadgeek]

Originally Posted by June

Dear Geektastic -

If we are moving at 17,500MPH, how come when I jump up in the air, I don't end up down the block?

Bewildered,
June

Because the Earth isn't *accelerating*. Remember that the Earth's rotation is constant velocity motion so we don't feel it and are perfectly justified in claiming that, at this moment, we are at rest. When you jump into the air while your vertical velocity, relative to the center of the Earth. temporarily exceeds 10 m/square second (temporarily overcoming the force of gravity) your horizontal velocity is 0. Since neither you nor the Earth are accelerating relative to each other, you land in exactly the same place as you started from. From your reference plane (the surface of the Earth) there is no force moving you horizontally and since a body at rest wil stay at rest until acted upon by another force, you jump up (which has a force acting on you vertically) and land in the same place (since no force is pushing you horizontally).


The situation would change if you were accelerating. To see this, let's do a little thought experiment.

You are on a plane, the plane is accelerating. You toss a ball up in the air, the ball will, in fact, land a bit behind you because the aircraft is moving relative to the motion of the ball. If, however, the aircraft is moving at a constant velocity then the ball will land at your feet.

Essentially, this is Einstein's General Theory of Relativity in a nutshell. If you are at constant velocity (what in technical jargon is known as the inertial frame) then you are justified in saying that you are at rest, no matter HOW fast you may be traveling. As long as whatever it is that you are traveling on maintains the same speed and direction, you can treat your environment as being at rest. It is only if you are accelerating that you will be aware of movement. One interesting side-effect of this is that gravity and acceleration turn out to be the same thing. Right now there is 1g of gravity pulling you toward the center of the Earth. We would be completely justified in describing us as falling toward the center of the Earth at 10 meters per square second. The reason we aren't all in the core of the Earth is that the electromagnetic force is MUCH more powerful than the gravitational force and the repulsion of effect of all the electrons in your body trying to keep away from all the electrons in your chair and in the floor is what keeps us from falling through the Earth. But right now, from a physical point of view, you are accelerating toward the center of the Earth. There's just something that prevents you from continuing the fall. If you were in a completely sealed box and were accelerating at 10 meters per sq sec. there is no experiment you could perform that would NOT lead you to conclude that you were not on Earth at 1 g.

Cheers
Aj