Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Half the size and mass, stronger force of gravity?

Tags:
  1. Jan 13, 2015 #1
    I've arrived at a non-sensical solution for a pretty simple thought experiment about gravity. Please let me know where my thinking is breaking down:

    Assume we have a cube of even density and a point mass on center of one of the sides. There is a force of gravity between the two of GmM/r^2 where r is half the length of one of the sides (the distance to the center of gravity of the cube). Now imagine the half of the cube farthest from the point mass disappears. It vaporizes. Now the mass of the cube is M/2, but the distance to the center of gravity is r/2. This would indicate an INCREASE in the force of gravity, but this is far from intuitive. Any ideas where I'm wrong?
     
  2. jcsd
  3. Jan 13, 2015 #2

    Orodruin

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    This is only true between point-like objects (or outside of spherically symmetric objects).
     
  4. Jan 13, 2015 #3

    Matterwave

    User Avatar
    Science Advisor
    Gold Member

    The shell theorem only applies for spherical distributions of matter. Treating a cube (or half cube) as a point is probably not going to get you great results. :)
     
  5. Jan 13, 2015 #4
    Thanks for the reply! I thought that may be the case - didn't realize that equation was based on that assumption.
     
  6. Jan 13, 2015 #5

    DaveC426913

    User Avatar
    Gold Member

    I'm not convinced the OP's question has been addressed.

    I suspect that his question will still exist, even he switches to using a spherical mass. I guess that's really up to the OP though.
     
  7. Jan 13, 2015 #6

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Your fundamental mistake (apart from using a cube rather vthan a sphere) is to assume that the centre of gravity of a hollow cube or sphere is not at its centre. In this case, a point on the surface is still a distance r from the centre of mass.

    So, the gravity on the surface reduces in proportional to the mass you have lost. This would be true of any spherically symmetrical reduction in the mass.
     
  8. Jan 13, 2015 #7

    Orodruin

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    If he uses a spherical masses instead he will need to specify what parts of the sphere disappears. If half the sphere mass disappears in such a way that the result is a new sphere, the force will decrease if applying the spherical solution. (Naturally, it will always decrease if using a correct approach.)
     
  9. Jan 13, 2015 #8

    jbriggs444

    User Avatar
    Science Advisor

    At the risk of re-hashing what has already been said. If you want to compare apples with apples then the smaller cube must still be a cube. It must have been cut in half in all three dimensions. So you slice away the far 1/2 cube. Then you slice off the outside of the flattened shape that remains. Result is a cube that is 1/8 the mass of the original and is at 1/2 the distance. An argument from symmetry indicates that it must therefore have 1/2 the attractive force (at its surface).

    The gravitational attraction of a cube of uniform density at its surface scales directly with the linear dimensions of the cube, just like the gravitational attraction of a sphere of uniform density scales directly with the linear dimensions of the sphere.
     
  10. Jan 13, 2015 #9
    I am pretty sure that the new r is not r/2, but something greater than that (although less than the original r).

    The decrease in M is much greater than the decrease in r. Meaning that gravity will decrease.
     
  11. Jan 13, 2015 #10
    If it was just sliced in half would be less gravity to the point on center face because y and z extremities cancel out, the point is closer to the center of mass so to speak.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook