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Volume change problem

  1. Dec 14, 2016 #1
    Mod note: Misplaced homework moved from General Physics, hence formatting template not shown

    Beginner here: Was recently given a homework problem:

    You have a scale (any kind), 2L beeker of water, a series of calibration weights, and one empty juice box. You want to accurately measure the volume change of the empty juice box if you suck most of the remaining air out of it.

    I kept thinking you want to just measure the difference in volume of the beeker with the box fully submerged, but how would the scale come into things?
     
    Last edited by a moderator: Dec 14, 2016
  2. jcsd
  3. Dec 14, 2016 #2

    mfb

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    How do you measure the difference in volume (as a number that you can write down) with just the beaker?
     
  4. Dec 14, 2016 #3
    well i guess that would not be accurate.
     
  5. Dec 14, 2016 #4

    mfb

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    If you just guess the amount of water displaced, it will be very inaccurate. How can you measure it?
     
  6. Dec 15, 2016 #5
    my friend is saying to fill the beaker with water and to put it on the scale, and zero the scale.Then you put a rod in the punch top of the juice box, and hold the box so that it is just submerged under the water, then to record the weight on the scale. Then he is saying to take the juice box with less air inside, and to do the same thing and record the weight, and the difference is the volume change. How would this work? The amount of water isn't changing, only the volume it occupies.
     
  7. Dec 15, 2016 #6

    mfb

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    If you fill the beaker completely, water will go out if you push something into the beaker.

    Holding the thing in while weighting, without filling the beaker completely works as well, but it is a bit harder to understand. Think about the force balance on the beaker (there are three forces acting on it).
     
  8. Dec 15, 2016 #7
    Would the three force be gravity, the normal force, and air pressure?
     
  9. Dec 15, 2016 #8

    mfb

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    You could consider air pressure but that wouldn't help much.

    What do you mean by "the normal force"?

    Gravity is one, correct.
     
  10. Dec 16, 2016 #9
    the normal force of the scale on the beaker, and the force of water in the beaker.
     
  11. Dec 16, 2016 #10

    mfb

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    What is "the force of water"?

    - Gravity
    - Force from the scale

    - Think about the person holding the empty juice box into the water. What are the forces involved?
     
  12. Dec 19, 2016 #11
    The person holding the juice box is adding the force needed to submerge the box in the water? And this contributes to the total balance on the scale?
     
  13. Dec 19, 2016 #12

    mfb

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    Right.
     
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