Celestia Forums

This may be in the "habitable" zone, but will never be habitable by us (as we would weigh more than 3X our normal weight on it's surface)

Really? I make it 'only' 2.2 gees; and that is with an unrealistic density.
Actually I think the figures we are given are probably too rough to make good estimates of density and gravity.

eburacum45 wrote:Really? I make it 'only' 2.2 gees

Here's my calcs (corrected):
V= GMm/r^2, so V ~ M/r^2 ... I think they said mass of 5x earth's and 1.5 x radius, so V = 5/1.5^2 x g --> 2.222222... g's.

eburacum45 wrote:Actually I think the figures we are given are probably too rough to make good estimates of density and gravity.

I never claimed it was a 'good' estimate. ...and I agree with you. From what I understand the observational technique used can only give a lower limit on the size anyway. It's still uninhabitable for us... and of course there's the little matter of getting there in the first place.

Chuft-Captain wrote:Here's my calcs:
V= GMm/r, so V ~ M/r ... I think they said mass of 5x earth's and 1.5 x radius, so V = 5/1.5 x g --> 3.3333333 g's.

There's an extremelly LARGE physics mistake there ! You fail my exam !

Oops! That's funny, I thought it looked a little bit wrong! It was just a 'little' mistake though.... it was only a little superscript 2.

Glad I wasn't sitting your exam Cham.

BTW: You fail my spelling exam.

extremelly

The mistake isn't just the exponent. It's also the symbol "V" that you used. "V" is commonly used to represent the gravitationnal potential, and the formula V = - GM/r is correct. However, it doesn't give the gravitationnal acceleration. You must write :

g = GM/r^2,

not V. So this is a conceptual error (not just a typo on the exponent), and that's why I've said it's an extremely large mistake.

And U = - GMm/r is the potential energy of the mass "m", in the field of "M", not to be confused with "V".

The mistake isn't just the exponent. It's also the symbol "V" that you used. "V" is commonly used to represent the gravitationnal potential, and the formula V = - GM/r is correct. However, it doesn't give the gravitationnal acceleration.

meeow!

Since this is the Physics and Astronomy forum, I had to point out the mistake. We can't let the confusion rise here.

While choosing an unconventional or even misleading letter for a physical quantity can hardly be called a "conceptional error", I completely agree with Cham that this was not a fortunate choice .

Notably since the gravitational Force ~ d V/ dr was associated with the widespread name for the potential (V) ...

While educators usually try to adhere to standardized notions for physical objects (for pedagogical reasons!), research scientists are always supposed to uniquely /define/ the objects they introduce, such that the associated acronym becomes more or less meaningless.


Bye Fridger