# Riddle me this

## Riddle me this

Any of you guys know how much planetary spin rate and a slightly lower air pressure than earths, could offset gravity? And say you had a planet 4x the size of earth, would the gravity also be 4x stronger, or does a lot of that depend on the size of ita molten iron core?

**Sir Pun**- Posts : 1621

Join date : 2013-01-30

## Re: Riddle me this

And just how much of a greater gravity do you think humans could realistic survive in? 1.5ish? Surely 2x would be too high for long term

**Sir Pun**- Posts : 1621

Join date : 2013-01-30

## Re: Riddle me this

Or is earths gravity even ideal for human habitation? What if it was .75% ?

**Sir Pun**- Posts : 1621

Join date : 2013-01-30

## Re: Riddle me this

Pun wrote:Any of you guys know how much planetary spin rate and a slightly lower air pressure than earths, could offset gravity? And say you had a planet 4x the size of earth, would the gravity also be 4x stronger, or does a lot of that depend on the size of ita molten iron core?

I don't know the answer to most of your queries (they lie far beyond the scope of my studies), however I can answer one question. Surface gravity (the gravity that one feels at the surface of the planet) can be calculated by the following:

g=(4/3)∏Gρr

Where:

g=gravity

4∏/3 = volume of planet

ρ=density of planet (m/v)

r=radius of planet (distance from center to point of observation, be it the surface or above the surface)

G=gravitational constant=6.67384x10^-11 N(m/kg)^2

So if you run this calculation for a planet with the same mass as the earth, but 4x the radius, you would end up with a very weak gravitational field (the same field you would encounter if you were orbiting the earth at a height of 4x earths radius).

**Bryant**- Admin
- Posts : 1452

Join date : 2012-01-28

Age : 29

Location : John Day, Oregon

## Re: Riddle me this

Bryant wrote:Pun wrote:Any of you guys know how much planetary spin rate and a slightly lower air pressure than earths, could offset gravity? And say you had a planet 4x the size of earth, would the gravity also be 4x stronger, or does a lot of that depend on the size of ita molten iron core?

I don't know the answer to most of your queries (they lie far beyond the scope of my studies), however I can answer one question. Surface gravity (the gravity that one feels at the surface of the planet) can be calculated by the following:

g=(4/3)πGρr

Where:

g=gravity

4π/3 = volume of planet

ρ=density of planet (m/v)

r=radius of planet (distance from center to point of observation, be it the surface or above the surface)

G=gravitational constant=6.67384x10^-11 N(m/kg)^2

So if you run this calculation for a planet with the same mass as the earth, but 4x the radius, you would end up with a very weak gravitational field (the same field you would encounter if you were orbiting the earth at a height of 4x earths radius).

Disregard that, I wasn't thinking clearly (as the math will show). 4x the radius of the earth with the same density would yield a gravitational field 4x that of the earth. Like I said, I'm not a physicist!

We can easily calculate this:

g=(4π/3)(6.67384x10^-11 N(m2/kg2))(5515 kg/m3))(4x6356800 m)=39.202 m/s2

gravity=(volume of sphere)(gravitational constant)(density of earth)(4xradius of earth)

39.2/9.8=4

**Bryant**- Admin
- Posts : 1452

Join date : 2012-01-28

Age : 29

Location : John Day, Oregon

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