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Question About Fluids

  1. Mar 26, 2008 #1
    Hello all,
    I have a very basic question about fluids. Here it is. There are two containers, one has alcohol in it and the other has oil. A steel ball is held down inside of each. A scale will weigh the steel ball as it is held down in the alcohol and then in the oil. Will the ball be lighter in the alcohol or in the oil or will the weight stay the same?

  2. jcsd
  3. Mar 26, 2008 #2

    Doc Al

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    Why does the ball have to be held down? How are you measuring the "weight"?
  4. Mar 26, 2008 #3
    Ok I knew I would not explain it right. The ball is held down in the fluid by a string, that sting is attached to a scale so the weight of the ball can be measured. So you are weighing the ball as it is suspended into the alcohol and then again in the oil. I wanted to know if there would be a difference in the weight as the steel ball is suspendind in different fluids.
  5. Mar 26, 2008 #4


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  6. Mar 26, 2008 #5
    Ok so I checked out the link. this is me thinking out loud and writing it down. I know that weight is w=mg. I also know that the mass and gravitational constant would not change. The density of the fluid being displaced would be different. So I am still unsure. I guess that the oil is lighter than the alcohol. I don't know I am still confused, any more help woudl be appreciated for my slow self. Thanks again.
  7. Mar 26, 2008 #6
    If just the metal ball was on the scale it would weight X. Once you place it in the two fluids, it will displace its volume of that fluid or the level of the fluid will rise. This displacement gives negative weight to the iron ball, y or z, depending on the density of each fluid. The new weight is then x-y or x-z. If X is less than y or z, we get a negative number and the ball will have negative weight and will float.

    We can trick nature and give the heavier metal ball a negative weight in either fluid if we pound it into the shape of a boat. It has the same weight on the dry scale. This time it will displace its weight instead of its volume. The fluid sees more volume per weight as though the iron has lost density, making it float. If fluid leaks in, the volume of the boat begins to lower as the fluid displaces it. This will increase its density until it sinks again.
  8. Mar 26, 2008 #7
    Ok these are freaking awesome answers and they are getting to the root of the question. Now I am trying to understand this. So the displacement gives negative weight to the ball. So now if there were no numbers or calulations on the question would the oil displace more volume than the alcohol? I know that the oil is lighter than the alcohol so it is less dense I guess. The thing that is tripping me up is that it is less dense, but more viscous. I am thinking too much on this. There are no numbers or calulations involved. So would the ball be lighter or heavier in one or the other or would it be the same. Again no numbers or calculations, just commen sense. What is the answer? I am a calulations kind of a guy. I took an aptitude test yesterday and I flew through the calcuation problems. I came to this one and thought about it too much. I just wanted to see what you guys thought.
    Last edited: Mar 26, 2008
  9. Mar 26, 2008 #8

    Doc Al

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    I presume that what's going on is that the metal ball is attached to a string and dangled into the fluid. The string is attached to a scale, which reads the tension in the string and is equal to the apparent weight of the ball.

    If the ball were dangled in air instead of fluid, the scale would read the full weight of the ball. But when the ball is in fluid, the upward buoyant force on the ball from the displaced fluid reduces the tension in the string.
    The fluid gives an upward buoyant force equal to the weight of the displaced fluid, thereby reducing the apparent weight given by the scale reading.
    The volume displaced equals the volume of the ball and thus is the same for any fluid. (But the weight of that displaced volume of fluid will depend on the fluid.)

    If you know which fluid is more dense, then that fluid will give the greater upward force (see Archimedes's principle). So does that mean the scale would read a higher or lower number?
  10. Mar 26, 2008 #9
    Ok so if the alcohol was more dense then it would give more upward force and the answer is the ball would weigh more in the oil than in the alcohol?
  11. Mar 26, 2008 #10

    Doc Al

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  12. Mar 26, 2008 #11
    Darn, I put that it would be heavier in alcohol.

    Wait stop the press...Please forgive me, but I am just trying to get a deep understanding.

    I just found that the specific gravity of Alcohol, ethyl (ethanol) is 0.787, and the sg of Oil, Castor is 0.959. Now my test question just said oil and alcohol. There were no specific numbers or details. So if the oil has a higher specific gravity than alcohol. The ball would weigh more in the alcohol?

    Is this correct. I know that earlier I said if the alcohol was more dense, but basically it comes down to the more dense object. I just assumed that alcohol was more dense than oil. So I guess the ball would be havier in the alcohol.


    Please tell me if this is correct.
    Last edited: Mar 26, 2008
  13. Mar 28, 2008 #12


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    The weight of an object is strictly a function of mass and gravity. By Niacidrac's wording, a bowling ball sitting on the floor would be weightless.
    In a buoyancy situation, it's only the apparent weight that is lowered. At least, that's the way that I interpret the situation. Experts are more than welcome to correct me.
    I'll use an example that is easily verified. Say that a guy weighs 80 kg in normal Earth gravity. That's what a bathroom scale will register if he stands on it. Now say that two of his buddies have to hold him up to get weighed because he's too drunk to stand on his own. They each support 20 kg, and the other 40 are still on his legs. The scale in that case will read '40 kg', but he still weighs 80.
  14. Mar 29, 2008 #13

    Doc Al

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    You're correct, of course. But in the context of this thread, naicidrac is using "weight" to mean apparent weight, which is all that a scale reads.

    (And if you dangled the ball in the fluid until it rested on the bottom of the container, the scale would read zero. :wink:)
  15. Dec 9, 2009 #14


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    Bottom Line........

    The answer to the question is that the ball suspended in the oil will weigh LESS than the same ball suspended in alcohol.

    The specific gravity of oil is greater than that of any alcohol. With that said, the greater the specific gravity of a liquid, the more it pushes against an object. This is why when put into the same container and let settled, the alcohol will "float" on top of the oil(The oil is pushing against the alcohol). The same goes for any liquid. If you were to add water to that same container, the water would settle at the bottom...oil in the middle...alcohol at the top. Water has a greater sg than oil, oil has a greater sg than alcohol, alcohol has a lighter sg than water and oil.

    Saltwater vs Freshwater.......freshwater will float on top of saltwater because saltwater has a greater specific gravity than freshwater. Using the same original question.....the ball would weigh less in saltwater. This is why you are more buoyant in saltwater when swimming.

    Water vs Oil.......Oil will "float" on top of water because water has a greater specific gravity than oil. SO.....the ball would weigh less in water.

    The simple rule to this type of question is this:
    The greater the Specific Gravity, the less an object will weigh.
    Last edited: Dec 9, 2009
  16. May 27, 2011 #15
    It's buoyancy of the ball acting upwards isn't it. Think "Archimedes principle".

    Viscosity makes no difference naicidrac. You have to have motion for that to make a difference, not a static situation.
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