### Orbits

• The attractive force of gravity can cause one object to orbit another.

• Qualitatively, one way to think about an orbit is...

1. gravity causes an object to fall toward the ground

2. 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.

• 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.

Notes

Updated October 1, 2007