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Old May 24th 2017, 01:42 PM   #1
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Fluid to increase force under same gravity and mass

if a beaker was filled with 200 cc of water and we tethered a ping pong ball to it's floor and that ping pong ball size is 10 cc .

Q1: what will be the weight of the beaker at that time ? (ignore the empty weight, ball and thread)

A1 : 200 grams

Q2: what will be the weight of the beaker at the first few milliseconds if we cut the thread ?

A2: still 200 grams ???

(for the next analysis , suppose that the ball is like balloon and we just popped it with a needle so the air inside it is free)

the way i see it is that it all depends on the total water pressure acting on the beaker floor and that force is 210 before and after the thread being cut or the balloon exploded (210 gram should be the multiply result of the beaker floor area x the water hight above it after the ball is submerged) but in the first case there was upward tension of the thread acting on the beaker while in second case, that tension was gone.
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Old May 24th 2017, 10:11 PM   #2
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when the string is cut ... the upward pull immediately ceases , so in the first millisecond the scale should show 210 ... thereafter a complex dynamic situation occurs , fluid rushing downwards , ball moving up...how quickly and in what way the scale returns to reading 200 must surely depend on the viscosity of the fluid , clearance between the sides of the beaker and the uprising ball, to name just two factors .
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Last edited by oz93666; May 24th 2017 at 10:29 PM.
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Old May 25th 2017, 11:35 AM   #3
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Originally Posted by oz93666 View Post
when the string is cut ... the upward pull immediately ceases , so in the first millisecond the scale should show 210 ... thereafter a complex dynamic situation occurs , fluid rushing downwards , ball moving up...how quickly and in what way the scale returns to reading 200 must surely depend on the viscosity of the fluid , clearance between the sides of the beaker and the uprising ball, to name just two factors .
Thank you , I tried to the experiment with home tools, can't confirm failure. The expected result makes sense even if it sounds odd . I been thinking to play with all possible factors to prolong the time where the load will be high. I agree, high viscosity won't help, I think of it as bubble in honey, maybe also surface tension and balloon shape and size.

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Old May 25th 2017, 02:20 PM   #4
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You need to be careful with the term "weight." If by "weight" you mean the downward force of the bottom of the beaker against the table it's sitting on" then I would agree that it will fluctuate due to dynamics of fluid flow, but in the steady state condition it will return to its starting value of 200 grams times g = 1.96 N. However, you refer to "weight" as something measured in grams, which causes me to think that perhaps you mean "mass." Remember that weight and mass are not the same thing. The mass of the system is constant.
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Old May 25th 2017, 02:26 PM   #5
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I am referring to the force downward (I don't know what to call it in this experiment). I may have excluded the multiplication of 9.8 as it is included at both sides of equations. I do expect fluctuations after the initial increase but my concern is the total net force

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