1. Not finding help here? Sign up for a free 30min 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!

Newton's law of universal gravitation

  1. May 13, 2016 #1
    1. The problem statement, all variables and given/known data

    Two 100-kg thrill seekers are diving into a neutron star. The neutron star’s mass is 1.1 solar masses and its radius is 12.0 km. For safety, they tie themselves together with a cord 10m long, so that as Melvin reaches the surface of the neutron star, Fred is 10.0 m behind him. Consequently, the pull of the neutron star on Melvin is very slightly stronger than the pull of the neutron star on Fred. However, due to the connecting cord, the two fellows accelerate towards the neutron at the same rate. By setting up free-body diagrams for both sundivers, find the tension in the connecting cord.
    media%2F98c%2F98cc668b-61da-43c9-a832-3379c9100733%2Fphp4dafYP.png



    Answer is 8.45 X 10^10 N


    2. Relevant equations
    ΣF = ma


    3. The attempt at a solution
    Mn = mass of neutron star
    m1 = mass of melvin
    m2= mass of fred
    R= radius of star
    d = length of cord

    Sum of forces on melvin
    (G*Mn*m1)/ R^2 - T = m1a
    Sum of forces on fred
    (G*Mn*m2)/ (R+d)^2 + T = m2a

    m1=m2 so
    m1a = m2a

    (G*Mn*m1)/ R^2 - T = (G*Mn*m2)/ (R+d)^2 + T

    T = (G*Mn*m1)/ 2R^2 - (G*Mn*m2)/ 2(R+d)^2

    Mn converted to kg = 2.19 x 10^30 kg
    R= 1200m
    G= 6.67 x 10^-11
    d = 10m
    m1=m2=100kg

    Plugging everything in im getting 8.35 x 10^13 N but the answer is 8.45x 10^10 N, what am i doing wrong
     
  2. jcsd
  3. May 13, 2016 #2

    TSny

    User Avatar
    Homework Helper
    Gold Member

    Your method and final formula for T look correct. Check the final calculation where you plug in the numbers.
    Note that the radius is 12 km, which is not equal to 1200 m.
     
  4. May 13, 2016 #3
    omg i just noticed 12 km is 12000 m not 1200 thanks
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Newton's law of universal gravitation
Loading...