Orbits

- The attractive force of gravity can cause one object to orbit another.
- Qualitatively, one way to think about an orbit is...
- gravity causes an object to fall toward the ground
- if the object is also moving horizontally sufficiently rapidly the ground curves away below it at just the same rate as it falls downward.
- orbiting is nothing more than a state of continuous freefall.
- When you are falling freely you have no sensation that gravity is acting upon you -- you are weightless.

- Quantitatively, Newton's Law of Gravitation ultimately explains orbits based
on the physics of gravity.
- Imagine swinging a ball on a string around your head.
- You feel a tug on the string due to the inertia of the ball. The tug is due to Newton's first law observation that the ball would prefer to travel in a straight line with constant speed.
- Your tug back on the string balances this ``centrifugal'' force and keeps the ball traveling in a circular path.

- For an orbiting celestial body the
*gravitational*force between two objects provides the tug that just balances the inertial ``centrifugal'' force.- Since there are simple formulae governing
- how the "centrifugal'' force depends on mass,speed, and separation and
- how the gravitational force depends on mass and separation.

These two formulae can be equated to derive the formulae governing circular orbits.

- Since there are simple formulae governing

- Imagine swinging a ball on a string around your head.

Notes

*Updated October 1, 2007*