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Fluid Mechanics (3)

  1. Jan 23, 2007 #1
    I don't understand the idea of apparent weight (see example below). Can someone explain? Thanks a lot!

    1) You need to determine the density of a ceramic statue. If you suspend it from a spring scale, the scale reads 28.4N. If you then lower the statue into a tub of water, so that it is completely submerged, the scale reads 17.0N. What is the density?
    The spring scale's reading is related to the tension T in the spring.
    The magniutde of tension T of the statue on the spring scale is equal to the magnitude of tension T of the spring scale on the statute (Newton's 3rd law)
    I actually have no problem figuring out the answer for this problem, but there is something that is confusing me! Is the reading of 17.0N the so-called apparent weight of the statute?

    I have been told numerous times that apparent weight means the normal force. But in this case the force is TENSION, not normal force, why would it still be called apparent weight? And even more confusing, as far as I know, tension isn't always apparent weight. For example, if you pull a box on the floor horizontally using a rope, that tension force is clearly NOT apparent weight. What is the actual definition of apparent weight? I am very confused...
    Last edited: Jan 24, 2007
  2. jcsd
  3. Jan 23, 2007 #2


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    17N is, indeed the apparent weight, in that some of the ceramic statue's weight is balanced by the buoyancy of the fluid.
    Thus, the force that the the spring needs to generate (as tension) in order to keep the statue at rest is less than the statue's actual weight.
  4. Jan 24, 2007 #3
    But apparent weight means normal force, right? (this is what I have been told for years!) For example, when you are free-falling, your normal force is zero, so your apparent weight is zero.

    Then, in this case, why would the tension force also be called apparent weight?

    How can I identify whether a particular force is apparent weight or not?
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