Q & A: Satellite Orbits

Q:
What stops satellites from dropping out of space?
- Chelsea (age 14)
Orange High, Orange, NSW, Australia
A:
Chelsea -

Man-made satellites don't fall out of space for the same reason that the moon (a big satellite) doesn't crash into the earth, or that the Earth doesn't crash into the Sun. Even though gravity pulls on the satellites, they are moving. If nothing pulled on them they would just keep going and end up going farther away from the earth. However, the earth's gravity pulls back down. The end result is that they move in a circle around the earth - in an orbit. This is kind of hard to describe in words, so try taking a look at this animated picture from The Physics Classroom.

The dot is like the satellite, and the earth is at the center of the circle. "a" is the direction that Earth's gravity pulls. "v" is the direction that the satellite would keep moving in if there weren't any gravity. That direction is constantly changing because of gravity, but the satellite never gets any closer to the earth.

-Tamara

(published on 10/22/2007)

Follow-Up #1: Hitting a baseball around the earth?

Q:
Then why when I hit a baseball with my bat, the ball falls back to earth? The ball goes in a straight line and it should move around the Earth instead of falling back according to your explanation. It doesn't make any sense.
- Eddie (age 27)
A:
You didn't hit it hard enough.  In order to achieve near earth orbit the ball must be traveling at least 7 km/sec.   See:http://en.wikipedia.org/wiki/Orbital_speed

LeeH

(published on 07/16/2010)

Follow-Up #2: Why the planets don't crash into the sun

Q:
Hi, I was looking up at the stars on a car journey back to home and a question popped into my head. If gravity is the attraction of objects with mass towards one another then what is the force that opposes gravity? For example, the earth must be attracted to the sun otherwise we would not orbit it but what stops us from colliding into it? Thanks, I hope I worded this correctly.
- Justin (age 21)
UK
A:

Hi Justin,

There isn't actually a force opposing gravity in the case you described. The reason the planets don't crash into the sun is actually due to Newton's laws of motion, not an opposing force.

Essentially, the planets are just going too fast to spiral into the sun. I think the best way to think about this is imagining a bowl with a ball spiraling around its upper edge. The ball is analogous to the planet orbiting around the sun. What actually causes it to fall towards the center of the bowl is the fact that it slows down, since there is a frictional force opposing its motion. Well, planets don't really slow down in their orbits since there's almost no friction in space. The result is that they just spin around the sun, never actually falling in.

Matt Joras

(published on 04/05/2012)