Avian, what's up with this new Earthlike planet?

[Avian(Admin)]

Originally posted by Bob Lentil
[b]Yeah, I understand how acceleration due to gravity is independent of the mass in question. My problem comes in with the any other force part. All of the examples you cited (A spaceship and a black hole. A baseball and the earth. Chris Squire and Spandex) are just more gravitational interactions, unless I misunderstand you.

Sorry for the confusion - in my haste I meant to type "gravitational acceleration" (g). It could be another gravitational force interaction, like a baseball and a black hole, or you skydiving on Mars. I meant to make the point that this is not special to the earth, or the particular gravitational acceleration near its surface, g.

Like I said, the force on each of these objects is different due to their different masses (it hurts a lot more to get hit with a falling bowling ball than a feather). Of course, if you impart the same force on them, they will have different accelerations. So, my statement should read:

But the acceleration doesn't have to be be gravitational acceleration (g) - it can be any acceleration by any external gravitational force. A spaceship and a black hole. A baseball and the earth. Chris Squire and Spandex.

Again, the constant things in these situation for each object (not going at significant relativistic speeds) is acceleration and mass. Each object will have a different downward force due to its mass.

To relate it back to what we were talking about..

Yes, that's why it has to go so fast around its orbit. Assuming it keeps the same mass but slows down, its orbit would decay and it would spiral into the sun. Likewise, if it maintains the same speed and gets heavier, the gravitational force between the sun and the planet increase, and you get the same result.

Fundamentally, a planet's period (time it takes to go around the sun) is not determined by its mass, for a planet of the sizes we are talking about. Just its distance from the sun. You can have a huge gas giant very close to the sun. You can have a tiny planet like Pluto way out in the outer regions. We neglect the force on the sun by the planet due to gravity because it's so small. If the planet, for some reason, slows down or speeds up due to an impact or something, its centripetal acceleration changes. It will migrate into another orbit with a different period that matches the new acceleration, unless that orbit brings it crashing into the sun or another object first!