1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Weight on [Planet] Question

  1. Sep 3, 2013 #1
    Neptune has a masss 17.2 times larger than that of Earth and a radius 3.88 times larger. A person weighing 176 lb on the Earth would weigh how much on Neptune?

    Earth's mass is 6x10^24; Earth's radius is 6.4x10^6. Neptune's mass is 1.032x10^26; Neptune's radius is 24832000. 176 lbs is 80 kgs.

    Weight (or Force) = G[itex]\frac{Mplanet\bullet Mperson}{r2}[/itex]

    G (universal) = 6.7x10^-11
    Mperson for Earth = 80 kgs
    Mplanet for Earth = 6x10^24 kgs
    r for Earth = 6.4x10^6

    Mperson for Neptune = ?
    Mplanet for Neptune = 1.032x10^26
    r for Neptune = 24832000

    Every time I try to solve for the mass of the person on Neptune, I keep getting around 70 kgs, which is 154 lbs. The correct answer according to my online homework system is 201.084048 lbs.

    First, I solve for Force on Earth. I plug all four values into the equation to get 785.15625 Newtons.

    Second, I then plug the answer above into the Force value to solve for Mperson on Neptune. To isolate Mperson I multiply both sides of the equation by r^2, divide both sides of the equation by G, and divide both sides of the equation by Mplanet. The answer I get is 70.02 kgs, which is 154.05 lbs. That is not the correct answer.

    I know that all I really need to do is divide 3.88^2 by 17.2, then multiply that value by 176, but I want to find out what I'm doing wrong. I don't want to use any shortcuts until I actually understand the long process.

  2. jcsd
  3. Sep 3, 2013 #2


    User Avatar
    Science Advisor
    Homework Helper

    You are going the long way around. Mperson on Earth=Mperson on Neptune and it's about 80kg. You don't need to solve for a different mass on Neptune. Mass doesn't change. Only the weight changes.
    Last edited: Sep 3, 2013
  4. Sep 3, 2013 #3
    Okay, I see what you're saying. I made a note in my notebook regarding universality of mass too, haha. I can't believe I skipped past that.

    So if I plug the value for Mplanet of Neptune, Mperson, r^2 of Neptune, and G, then I will get around 897.06 N. I just convert this into pounds right?
  5. Sep 3, 2013 #4


    User Avatar
    Science Advisor
    Homework Helper

    Sure, but as you said, just using ratios is the easy way to do it.
  6. Sep 3, 2013 #5
    Thank you!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted