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

Physics of a milk bottle

  1. Oct 19, 2010 #1
    1. The problem statement, all variables and given/known data
    An old fashioned non-homogenized milk bottle is shaken up so the cream and milk are uniformly mixed. The bottle is then placed down, and the cream and milk begin to separate. Does the bottle weigh more or less when it is being carried or just after it is put down? Estimate the difference if any.

    2. Relevant equations
    As far as I can see it's a purely qualitative problem.

    3. The attempt at a solution
    I think the answer may be that it weighs more when it is put down, as the milk has momentum as it separates downwards which is greater than the momentum of the cream upwards as the milk has greater mass and they are both separating at the same speed.
    No idea how to estimate the difference in weight though...

    Anyone have any ideas about this problem?
  2. jcsd
  3. Oct 19, 2010 #2
    I can't really understand.
    Is the bottle closed ? If so, nothing goes in, nothing comes out, how can it weight differently ?
  4. Oct 19, 2010 #3
    I think it weighs more just after it is shaken than when it is put down.
    The cream starts separating just after we stop shaking the bottle, cream being less denser than milk starts rising upwards thereby exerting a force downwards which adds upto the weight of the bottle for a moment.
  5. Oct 19, 2010 #4
    Yeah, ok.
    But the weight is just an acceleration (Weight = g*m)
    (A hint for your solution)

    As soon as the convective motion in the milk mass has stabilized (very soon I guess), there is no more acceleration, so the weight must be it's rest weight.

    And when exist, the difference is really little.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook