- #1

fawk3s

- 342

- 1

## Homework Statement

Is it possible to calculate the mass of the Milky Way knowing only the speed of a planet in a circular orbit? I fail to see how, but if yes, then how?

Thanks in advance

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- #1

fawk3s

- 342

- 1

Is it possible to calculate the mass of the Milky Way knowing only the speed of a planet in a circular orbit? I fail to see how, but if yes, then how?

Thanks in advance

- #2

SHISHKABOB

- 541

- 1

The relationship between a planet and the star its orbiting has no connection to the mass of the Milky Way.

- #3

fawk3s

- 342

- 1

I did type planet, indeed. Let it be a star then for clarity's sake. Think you get the point.

- #4

SHISHKABOB

- 541

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so if F

can you take it from there?

- #5

fawk3s

- 342

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so if F_{net}= F_{g}

can you take it from there?

Well, in a circular orbit,

F

You are given only the speed v. The problem is how to get rid of the radius (r). Dunno how, and if possible.

- #6

SHISHKABOB

- 541

- 1

What's the exact problem and what values are you given?

- #7

fawk3s

- 342

- 1

Find the mass of the Milky Way, when an orbiting star's speed is 220 km/s.

- #8

fawk3s

- 342

- 1

- #9

BruceW

Homework Helper

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Exactly. If you don't have the radius, then you can't get the speed. You would need to find the radius some other way, or introduce other assumptions.Well, in a circular orbit,

F_{centripetal}=F_{g}

You are given only the speed v. The problem is how to get rid of the radius (r). Dunno how, and if possible.

- #10

fawk3s

- 342

- 1

What kind of assumptions could be introduced here?

- #11

BruceW

Homework Helper

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- #12

fawk3s

- 342

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for example, you could assume that the star fits the typical trend of other stars in orbit around the milky way, then you could work out the radius of the star.

Could you elaborate on this please? Seems interesting, but dont think I follow. What kind of trend and how do you work out the radius from that?

Thanks in advance

- #13

BruceW

Homework Helper

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But your original problem was to find the mass of the milky way given only the orbital speed of the star, so this method would not be any help.

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