Gravity and Orbits
Name: Akhila S.
Date: Thursday, September 12, 2002
I was asked by my students about why the earth and all
the planets do not fall into the Sun, if it is the Sun's gravity that is
keeping them in place. The explanation I gave did not seem to carry
conviction. In any case, I have not been able to visualize it clearly myself.
The second question is on the elliptical orbits as well. Why that shape?
According to Newton's first law, an object in motion remains in motion IN A
STRAIGHT LINE, unless acted upon by a net force.
The planet is moving in a straight line, but pulled from that straight line
by the gravitational force due to the sun. The planet is now moving in a
new straight line, but once again, is pulled from that line by the
gravitational force due to the sun . . . . . .
Go up on a mountain with a bunch of balls. Toss one. It hits the ground
near you. Throw one, it goes further. Fire one, it goes further yet. Now
consider throwing a ball so hard, that the amount it drops matches the
curvature of the ground. It would never hit the ground! This is called an
orbit. It would be circular.
Now, if you threw it just a bit harder, it would overshoot the circle, but
still orbit. This shape is an ellipse. If you threw it faster still, it
could escape from the earth's orbit into the universe and the path would no
longer be a closed figure like a circle or ellipse.
---Nathan A. Unterman
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Update: June 2012