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Homework Help: Upthrust/ how does the weight of a sphere change in water?

  1. Jan 4, 2016 #1
    1. The problem statement, all variables and given/known data
    See image attached. Oh it's part c by the way.

    2. Relevant equations
    maybe upthrust=weight of water displaced... None really relevant.

    3. The attempt at a solution
    I thought the balance would initially go up as the sphere entered the water, (but maybe slightly less than the weight of the beaker+sphere because up thrust is acting on the sphere) because the weight of the sphere would be greater than any opposing forces i.e upthrust because the sphere is accelerating. Then when the ball was rising to the surface I would assume the balance reading would be just the weight of the beaker as upthrust will be greater than the weight of the sphere ( or maybe you could calculate the upthrust then take that away from the weight of the beaker?)Then when the sphere is floating on the surface the balance would read the weight of the sphere+beaker. Is any of this sort of right?

    Attached Files:

  2. jcsd
  3. Jan 4, 2016 #2


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    Do you mean before or after the sphere has become fully immersed?
  4. Jan 4, 2016 #3
    Um before
  5. Jan 4, 2016 #4


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    I think I would break this into stages, eg. ball out of water, ball entering water, ball falling in water, ball stationary in water, ball rising in water, ball floating on water, (*)
    At each stage consider at first only the main forces such as weight and buoyancy to establish the general pattern.
    Then you can add considerations of acceleration to see how they affect it, perhaps qualitatively at first, then calculate some values if you can.

    There may be other factors you could think about (* and stages), but I'll not mention them unless you do.
  6. Jan 4, 2016 #5


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    The "weight" of sphere, partially submerged in water, is its "usual" weight, out of water, minus the weight of the water displaced.
  7. Jan 4, 2016 #6
    Show us what you did in parts a and b. Your conclusion regarding the final state in part c is correct. It seems to me, the missing piece of the puzzle is doping out the situation at the instant that the ball has come to a stop under the water. Once you have that, you should be able to fill in all the blanks.

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