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Introductory physics hw

  1. Oct 1, 2013 #1
    Hello, sorry if this is an easy question, but I don't quite get it.

    Here it is:

    Planet X is home to an alien culture called the Xinbar. The planets diameter is twice as large as the earth's and made out of the same stuff (i.e., the densities are the same). The Xinbar are made out of the same stuff as humans and weight the same on Planet X as we do on Earth. The equation for gravity is shown below, which reduces to mg near the earth's surface.

    (note: I wasn't sure how to put in an equation since i'm new here, but its just the gravitation equation = mg)

    (a) What is the volume of the earth?
    (b) What is the volume of Xinbar? Determine the mass.
    (c) What is the gravitational constant g on xinbar?
    (d) We can model humans as cylinder roughly 6 ft tall and 2 ft wide. the Xinbar have roughly the same proportions, estimate how tall they are.
    (e) Estimate how much a human would weigh on Planet X.


    For part (a) I just used the equation for volume of a sphere using the earths diameter. Not sure if this particular problem suggest me to do it a different way.

    I did the same for the first part in b, but not sure how to calculate the mass.

    Any help would be appreciated. Thank you!
     
  2. jcsd
  3. Oct 1, 2013 #2
    If you want the volume of a sphere I think you'll probably be using a formula that has the radius of the sphere and not the diameter.

    The question mentions the densities of the two planets so it'll probably be useful to know that the density of an object is mass per unit volume. Do you have formula that's got density, mass and volume in it? If not, you can work it out from the definition of density. You'll also need to know the mass of the Earth.

    The question says that the densities of the two planets are the same. So what I'd do is write down an expression for the density of the Earth and set that = the density of the other planet. With a little algebra you should be able to find the mass of the alien planet.
     
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