Ask A Scientist© Physics Archive

name         Rocky
status       student
age          15
Question -   When the planets orbit around the sun, the sun has a 
gravitational pull so they don't go flying off into space.  With the 
power of the sun's gravitational pull, why don't the planets just come 
crashing into it?
-----
There is a nice balance between the direction change due to the 
gravitational attraction, and the motion of the object.  Think of from 
events you know on earth.  Go up on a mountain.  Toss a rock.  It lands a 
little bit down range.  Now throw a rock.  It goes further down range.  Get 
a gun and shoot the rock.  It goes further yet.  Each second the rock was 
in the air, though, it was falling toward the earth, and the earth caught 
it.  Now, what would happen if you threw the rock so hard, that it would 
fall at the same pace as the earth curves out from underneath it.  It would 
fall the entire time, but instead of hitting the earth, it would just keep 
on missing it, and fall around the earth.  That is the idea behind orbital 
motion.

Nathan A. Unterman
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