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Homework Help: Calculating the radius of a planet based on its mass

  1. Oct 18, 2011 #1
    1. The problem statement, all variables and given/known data
    The Earth is 6371km in diameter and this is equivalent to 1 mass.
    If I increase the Earth's mass to 2x what will be its diameter?
    What is the formula to compute this?

    2. Relevant equations

    3. The attempt at a solution

    PS: This is my first post and I wish to say hello to everyone!
    Last edited: Oct 18, 2011
  2. jcsd
  3. Oct 18, 2011 #2


    User Avatar

    Staff: Mentor

    Welcome to the forum.

    It is probably good enough to assume the earth to be spherical and of consistent density throughout.
    Last edited: Oct 18, 2011
  4. Oct 18, 2011 #3
    Hi. I'm not quite sure of the intentions of your comment. Can you please explain it?
  5. Oct 18, 2011 #4


    User Avatar

    Staff: Mentor

    What's the relationship between radius, volume, density, and mass for a sphere?
  6. Oct 18, 2011 #5
    This thread is getting weirder and weirder...
    Is my post in the right secion of the forum?
  7. Oct 18, 2011 #6


    Staff: Mentor

    The people responding in this thread are trying to get you to think about this problem, and are asking questions that are directly related to it.

    You posted in the Engineering, Comp. Sci, & Technology subforum - a better place is the Precalculus subforum. I am moving the thread to that subforum.
  8. Oct 18, 2011 #7
    I know they are trying to help me which I thank, unfortunately they are not giving me any help at all. I need answers not counter-questions. My math skills are next to zero but even though I can put formulas to work as long I have them. The density is NOT relevant for this formula and I suspect that the volume follows the same path.

    I simply need to know the formula to compute the following:
    The earth is 6000km and equals 1x mass what would be its radius if the Earth's mass would double?
    How can astrophysics know that Mars in only 10% of the Earth's mass while having 53% of the Earth's diameter?
  9. Oct 18, 2011 #8


    Staff: Mentor

    At Physics Forums we do NOT provide answers. Here's an excerpt from the Physics Forums rules, which you agreed to abide by when you started your account (emphasis added).
    Meaning that volume is not relevant in this problem? It is very relevant here.

    What formulas do you know or can you find? One that would be very useful is the volume of a sphere.

    Knowing the diameter and density of Mars, they can determine the mass for that planet.
  10. Oct 18, 2011 #9


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    Gold Member

    Well, the density is probably derived, since we can't measure it directly.

    But since Mars has two moons, we can definitely determine its mass. Then, along with volume, that allows us to compute the density.
  11. Oct 18, 2011 #10


    Staff: Mentor

    By "knowing the density" I meant that Mars, Earth, Venus, and Mercury are all rocky planets (as opposed to the gaseous ones like Jupiter and Saturn), with roughly similar densities.
  12. Oct 18, 2011 #11
    I apologize then, I had no idea this forum was that useless, at least for me that is. I used to those forums where I give direct answers and where I get direct answers, you know were everything is direct; where knowledge-sharing really happens. Without riddles or ghostly voices. That's why I thought strange about the first posts I got in my thread. Now all is crystal clear. I honestly never came across a forum where you couldn't simply give solutions away... it sounds SO ODD!

    Well ok... so it is... too bad that won't change a thing.

    I know I can get such formula on Wikipedia in no time (thank god there are sites with direct answers) but that would still be no good for me. It is not just a matter of having all the pieces but knowing how to put all the pieces together.

    And the formula for that is? I know, that would be against the forum's policy.
    Does that formula at least have a name so I can web browse it elsewhere?

    I can't help it, I have to say this:
    This forum is not teaching anything it acts like a brain stimulating machine where it attempts to recall that small piece of knowledge from our brains which was sent to oblivion by time. This is the reason why I say this forum is useless to me as I'm here to gather and share knowledge, I'm not exactly here to remember anything. Can't remember what I never knew. I'm not here to think but to collect and give away. The thinking part is already taking place in my project and that consumes already too much time.

    5 posts where made with the overall of 0% efficiency. Not that persons are obligated to help me or that are guilty of my ignorance but when I post something to someone I really try to help. I mean really helping. I put myself on that person's shoes and see if what I say is indeed helpful. I understand why Gneill's and NascentOxygen's posts where not that helpful because their posts were and are castrated by the forum rules.

    This is so ridiculous. LOL! I honestly just laugh out loud! I mean, a forum that is dedicated to a complex subject such as physics which can't be talked upon directly. You can present me the best explanation of the world of why the forum possesses such rules that I still won't be able to conceive it. I believe that there might be laws involved in it which makes the whole thing even more hilarious!

    Well, I guess I need to move along and find another solution :).
    I apologize you all for wasting your time in vain. Honestly. And it is all my fault as I did not read the forums rules.
    Last edited: Oct 18, 2011
  13. Oct 18, 2011 #12


    Staff: Mentor

    Well, different strokes for different folks. I'm sure you can find any number of sites out there that will spoonfeed you the answer to your question, without the need for you to do any thinking whatsoever. That, as you are finding, is not the intent or purpose of this site. We are very willing to help you work your problem, but you have to be willing to put in some effort. Because I have seen none on your part, I am closing this thread.
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