How can you accurately measure the volume change of an empty juice box?

[charles bigby]

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?

 

[mfb]

How do you measure the difference in volume (as a number that you can write down) with just the beaker?

 

[charles bigby]

How do you measure the difference in volume (as a number that you can write down) with just the beaker?

well i guess that would not be accurate.

 

[mfb]

If you just guess the amount of water displaced, it will be very inaccurate. How can you measure it?

 

[charles bigby]

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.

 

[mfb]

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).

 

[charles bigby]

Would the three force be gravity, the normal force, and air pressure?

 

[mfb]

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

What do you mean by "the normal force"?

Gravity is one, correct.

 

[charles bigby]

the normal force of the scale on the beaker, and the force of water in the beaker.

 

[mfb]

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?

 

[charles bigby]

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?

 

[mfb]

Right.