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The spring buoyancy

  1. Oct 26, 2011 #1
    Hello,world
    [PLAIN]http://img507.imageshack.us/img507/4446/samar.jpg [Broken]
    1-How does this spring scale work?

    2-Where does this increase in the reading of the normal balance(scale) come from?
    Does it come from the rise in water level in the tank?Okay water already has aconstant mass

    Does it come from the weight of the ball?I don't think so.

    3-why does the apparent weight decreases as the ball goes down into the fluid(by increasing depth)??may be because the buoyant force increases..
    the buoyant force is different in case(2) and case(3) although the ball already displaced the same volume of water-so buoyancy should be the same.can you realise my problem here?

    4-In case(3)
    The spring reads zero
    which means that Fb=Fg
    so the body should suspend in water instead of sinking ...or may be the spring is exerting a certain force to maintain the ball in this position.may be this question is related to the first one which is concerned with the mechanism of the spring balance

    5-can we know from this picture if the spring has a smaller-equal-bigger density than that of water?or it just doesn't indicate?
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Oct 26, 2011 #2
    1.- When the ball is at the bottom, the weight of it is being held by the bottom scale and the top feels nothing.

    2.- In the middle, the ball is being pushed up by a force equivalent to the weight of the volume of water displaced...so the water is pushing up and so the scale at the bottom feels such reaction and the scale at the top feels the help

    3.- Same as 2, except that there is water displaced.

    4.- ...

    From the readings, I gather this ball definitely sinks in water. In other words, the long line from the top scale down to the ball could be a rope and it would still work this way.
     
  4. Oct 28, 2011 #3
    Do you mean that the normal balance measures the upward force only,either the normal force of the water tank or the buoyant force?
     
  5. Oct 28, 2011 #4
    When the ball is not at the very bottom, the balance reports the weight of the water plus the buoyant force.
     
  6. Oct 30, 2011 #5
    first what do you mean by "the scale at the top feels the help"?which help?
    how does the normal balance report both the weight of water plus the buoyant force??
    may be due to the increase in pressure at the base f the tank due to rise in water,this indicades an increase in the total force affecting the base of the top ,thus increasing the reading of the normal balance.
    Is it because water rises in the tank?does it measures the weight of water + weight of water that rises?how about the weight of this suspended ball?
    but this doesn't tell me why the buoyant force is different in case(2) and case(3) although the ball already displaced the same volume of water-so buoyancy should be the same

    4-In case(3)
    The spring reads zero
    which means that Fb=Fg
    so the body should suspend in water instead of sinking ...or may be the spring is exerting a certain force to maintain the ball in this position.may be this question is related to the first one which is concerned with the mechanism of the spring balance

    I still don't know the answer to this question.

    if the body floats,then it would suspend in case 4 instead of sinkning in the bottom (Fg=Fb) true?or I still don't understand how this ballance works ?
     
  7. Oct 30, 2011 #6
    Let's supposed the you can reach the ceiling of the room you are in and you place a balance on the floor and you weigh yourself...how much do you weigh?

    Now, raise your arms and push against the ceiling....how much does the balance reports?

    It's the same when the ball is mid-water...because of Archimedes, the water exerts an upwards force on the ball, but because the ball will not float and go up, instead it stays where it is, then the balance under the water reports the weight of the water, plus the reaction force of pushing the ball up that will not move.

    does this help?

    Oh, and...

    3.- Same as 2, except that there is water displaced.

    I meant to say:

    3.- Same as 2, except that there is LESS water displaced.
     
  8. Oct 30, 2011 #7
    It would report much force,my weight + the force that i apply on the ceiling
    correct?
     
  9. Oct 31, 2011 #8
    yeap
     
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