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Physics question

  1. Jul 1, 2005 #1
    I am not sure if this is the right place to ask this. A couple us at my office were trying to decide if you have a fish bowl half filled with water sitting on a scale, then you add 3 goldfish does the weight increase?
    Last edited: Jul 1, 2005
  2. jcsd
  3. Jul 1, 2005 #2


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    Yes. Goldfish weigh something. It doesn't matter if they are swimming or not.
  4. Jul 1, 2005 #3


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    Think of it this way...Are you adding mass to the system of what is being considered? If you add mass...............
  5. Jul 1, 2005 #4
    I think your co-workers are confused with the thought of "bouyancy".
    This will be simple, have you co-workers do this:

    Take a large glass, fill it halfway with water, place it on the office postal scale and note the reading. Now, place several ice cubes in the water(not so many that they touch the bottom of the glass) Note that the scale reading is higher.

    Then, take that glass with the water and ice cubes and place it into your office refrigirator's freezer until the contents of the water and ice cubes freeze solid. Take it out and place it on the scale. That reading, and the one with water and ice cubes will be the same(minus collective condensation of course)
    Last edited: Jul 1, 2005
  6. Jul 1, 2005 #5

    Doc Al

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    Of course the weight measured by the scale increases, as others explained. (Otherwise things would be really strange!)

    Perhaps the confusion comes from the buoyant force that the water exerts on the fish. The net force on the fish (their weight minus buoyant force) is zero. (So, in a sense, they "weigh" less in water.) But that buoyant force comes at a price: the water level rises, increasing the water pressure and thus the force that the water exerts on the bottom of the bowl (and that the bowl, in turn, exerts on the scale).

    Another way to think of buoyant force is this. If the water exerts an upward force on the fish (which it does), the fish in turn must exert an equal downward force on the water. That added downward force on the water ends up as an additional force on the scale that exactly equals the weight of the fish.

    The bottom line is that the force that the scale exerts on the bowl (which is what the scale measures) must equal the total weight of the bowl and everything in it.
  7. Jul 1, 2005 #6


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    I wonder if the displacement might be confusing them. If the bowl was full of water, adding fish would dump some of it out. Obviously, when there's enough bowl to hold it all, that doesn't happen.
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