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!

Gravitational Force Units Question

  1. Mar 12, 2013 #1
    I just have a question concerning a previous thread about gravitation.

    The formula is from post 19 at https://www.physicsforums.com/showthread.php?t=635188&page=2


    v= dr/dt

    a = dv/dt

    multiply by dv/dt by dr/dr:

    a = (dr dv)/(dt dr) = v dv/dr

    This gets you to the first step:

    v dv/dr = -G (m1 + m2) / r2


    My question is: Why is it possible to set a = v dv/dr = -G (m1 + m2) / r2 ?

    On the left side there are units of acceleration (m/s^2) and on the right side is the gravitational force (kg*m/s^2). Shouldn't the units at all time be the same on both sides?

    Thanks very much for an answer.

    Regards, Philipp
     
  2. jcsd
  3. Mar 12, 2013 #2

    rbj

    User Avatar

    hey philipp, please use [itex]\LaTeX[/itex]. it ain't hard.
     
  4. Mar 12, 2013 #3

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    Are you sure? Look up the units of G and check that again.
     
    Last edited: Mar 12, 2013
  5. Mar 12, 2013 #4
    Thanks for the reply.

    Well usually -G (m1 + m2) / r2 = F = m * a

    This is not equal to just acceleration as in a = (dr dv)/(dt dr) = v dv/dr = -G (m1 + m2)
     
  6. Mar 12, 2013 #5

    Doc Al

    User Avatar

    Staff: Mentor

    No. Gravitational force requires two masses multiplied, not added: Gm1m2/r2

    The left side of your equation has units of acceleration, not force.
     
  7. Mar 12, 2013 #6
    aaa, thanks. so silly from me :)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Gravitational Force Units Question
  1. Unit of force question (Replies: 2)

  2. Gravitational Force (Replies: 2)

  3. Gravitational force (Replies: 9)

Loading...