# Orbit

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#### chiddler

##### Full Member
10+ Year Member
A new planet is discovered 100 R(earths) from Earth. With this information, what else can we determine about the planet?

A. Both mass and orbital velocity.
B. Mass only.
C. Orbital velocity only.
D. Nothing.

Need help with the question. Thanks!

#### SaintJude

##### Full Member
Well, we definetely can't find mass. We would be able to find mass perhaps if we knew the strength of the gravitational force perhaps, but it's not given.

I figured that the we could find orbital velocity, b/c I figured it was equal to the escape velocity. This was just something I knew out of practice (especially b/c you, chiddler, have listed a few escape velocity problems--so thanks?)

And indeed wiki:

Assuming that gravity is the only significant force in the system, this object's speed at any point in the orbit will be equal to the escape velocity at that point (due to the conservation of energy, its total energy must always be 0, which implies that it always has escape velocity; see the derivation above).

And, in case it's rusty: escape velocity is only dependent on the Earth's Mass:

#### pm1

##### Full Member
A new planet is discovered 100 R(earths) from Earth. With this information, what else can we determine about the planet?

A. Both mass and orbital velocity.
B. Mass only.
C. Orbital velocity only.
D. Nothing.

Need help with the question. Thanks!

I don't know it this is correct. But how I did it I equaled centripetal force to gravitational force. Then, you can cross out the planet mass. Now you have g = v^2/r
However, your g will change since R is now 100R it will be g/10^4. Then you can find v^2 from there.
I don't know if that's right but got me to the answer.

10+ Year Member
7+ Year Member

#### chiddler

##### Full Member
10+ Year Member
alrighty

so i was thinking a = v^2/r? acceleration doesn't tell us anything.

which is, of course, false.

you're welcome stjude

#### Tatiana3325

##### Membership Revoked
Removed
A new planet is discovered 100 R(earths) from Earth. With this information, what else can we determine about the planet?

A. Both mass and orbital velocity.
B. Mass only.
C. Orbital velocity only.
D. Nothing.

Need help with the question. Thanks!

Um, it's so far away that it might not exist anymore?

I have no idea but I'm sure the know it all's at NASA can answer your question. They think they're soo smart.

#### dmf2682

##### Membership Revoked
Removed
Well. If they observed the motion of known objects with this new information they could figure out the mass. For example, each of the planets causes a certain amount of wobble in the sun, as well as having an effect on the other planets' orbits. From that you could figure out gravitational force between them and thus mass.

But it's asking based on the information given alone, so it'd have to be just velocity.