MadSci Network: Astronomy

Re: Why is there no gravity in outer space?

Date: Sat Mar 18 20:52:57 2000
Posted By: Joseph Lazio, Radio Astronomer
Area of science: Astronomy
ID: 952652850.As

Toss a ball. What happens? The ball leaves your hand and travels a short distance, in a parabolic arc, before hitting the ground.

Throw the ball. Now it travels farther before hitting the ground. Fire the ball from a cannon, and it travels even further.

As the ball leaves your hand (or the cannon), it has some horizontal motion. As soon as it leaves your hand, it also starts to fall. Where the the ball lands is determined by how far the ball can travel horizontally in the time it takes for it to fall from your hand. (Many people are surprised by this fact---if you could throw one ball horizontally at some speed while simultaneously dropping a second ball from the same height, both balls would hit the ground at the same time [though obviously in different places].)

Continuing our thought experiment, we remember that the Earth is a round object. In our ordinary lives, the curvature of the Earth's surface is so slight it is immaterial. However, suppose that the ball can be fired horizontally at a great speed. If there was no gravity, the ball would simply travel horizontally. Because the Earth's surface is curved, the distance between the ball and the Earth's surface would increase as the Earth's surface curved away from the path of the ball. In fact, there is gravity, so as the ball is travelling horizontally, it is also falling toward the ground, but the ground is curving away from the ball.

If the speed of the ball is great enough, it never hits ground. As it falls toward the ground, it travels far enough horizontally that the ground has curved away from it. It is forever orbiting or falling around the Earth.

Astronauts in orbit float not because there is no gravity, but because they are falling toward the Earth but never hitting it. (The same is true of the Earth orbiting the Sun.) This condition, known as free fall, is quite different than our ordinary experience standing on the Earth. Nonetheless, it can be duplicated. Many amusement parks now have rides that allow one to experience free fall (or near free fall) for a few seconds. NASA has an airplane, affectionally known as the Vomit Comet, in which scientists can conduct tests in free fall without having to go all the way to space.

(My description of throwing a ball has neglected one important factor---air resistance. In any real experiment like I've described, air resistance would be quite important. That's why rockets take off upward. The objective is to get above much of the atmosphere before starting to angle the engines horizontally and throw the rocket horizontally. Also, by travelling away from the Earth, Earth's gravity is less so less of a horizontal throw is required to make the rocket orbit the Earth.)


Re: Why is there no gravity in outer space?
I am a sixth grade teacher and one of my students asked me this question.  
She literally stumped me because I had always heard and excepted that 
there was no gravity in outer space, I never questioned why.  If you could 
enlighten me, even if its very technical, then I can break it down for my 
students.  Thanks!!

I'm not posting this question, because we might want to iterate on it and
because I want to ask a few questions.

Why does your student think that there is no gravity in outer space?  If
there is no gravity, why does the Earth orbit the Sun?

	Joseph Lazio, Admin MadSci Network
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