# Daughter needs help with crazy physics instructor

1. Feb 5, 2004

### thundermist

My daughter has a sadistic physics instructor who just LOVES to see kids fail. He assigned the following problem to 50 students, but "guarantees" there are no where near 50 answers. Since every student must have a different solution, only a small portion of the class will have a chance of passing. Whoever gets the workable solutions first are the ones who will pass. Not fair, but condoned by the school.

Any suggestions would be greatly appreciated

********
The problem:

A large glass cylinder (approx. 4 feet tall) is filled with water to about 7 inches from the top .... 3 or 4 inches below the water line, a blue line has been drawn on the cylinder .... several inches of small copper wire is folded loosely and dropped into the water .... the goal is to raise the copper wire above the blue line (even if only for a second)

The rules:
-- magnetism can't be used
-- anything inserted into the water cannot be exposed to the air and must be totally submerged (for example, you couldn't have a fish hook on a line)
-- anything you add to the water cannot dissolve or alter the copper wire
-- anything you add to the water cannot "significantly" change the composition of the water
***********
Thanks in advance for any suggestions
~Mike~

2. Feb 5, 2004

### Njorl

Train a monkey to jump in and hold the wire up without surfacing.

Hit the cylinder with a hammer, shattering it, so that the blue line falls to the floor. Smash the glass until the entire line is below the wire. In some reference frame, the wire is seen as rising.

Pour in mercury. Copper floats in mercury.

Whirl the cylinder in the verticle plane such that the cetrifugal force exceeds g. When the cylinder is upside down, the wire at the bottom is above the line.

Take the cylinder to a zero g environment. Brownian motion will eventually put the wire above the line.

Pour the water out, reach in and grab the wire.

Blast the bottom with supercooled helium so that a sheet of ice forms on the inside. AS the sheet begins to melt, it will detach from the bottom and begin to float, with the copper wire.

This took just a few minutes. I have no doubts I could do more than 50. You must be leaving something out, or the teacher is trying to be intimidating.

Njorl

3. Feb 5, 2004

### NateTG

Put a column in the cylinder underneath the wire.

Carbonate/Oxiginate or disolve a different gas in the water. The bubbles forming on the wire will lift it.

Gently place the copper wire on the surface - surface tension will hold it up.

Take a solvent, wipe the blue line off of the cylinder with a sponge and place it underneath the cylinder.

Freeze the water and invert the cylinder.

Place the cylinder in a sufficiently strong static electric (not magnetic) field.

Attach the copper wire to a cartesian diver.

4. Feb 5, 2004

### Rog

put a lid on it and turn it upside down

5. Feb 5, 2004

### wasteofo2

You could attach something that floats to the bottom of the copper wire.

You could coil a spring and put it under the copper wire and let the copper wire be propelled above the line (might need a large spring).

There would be hundreds of different battery powered machines which you could place under the wire to lift it.

Unless her physics teacher decides to come up with a new set of circumstances, I see no reason why everyone who tries (or just visits this forum and asks what to do) shouldn't pass. And hey, if her teacher decides to come up with new conditions and won't pass her, she can just take the assignment sheet to the principle and explain why she should pass.

Last edited: Feb 5, 2004
6. Feb 5, 2004

### LURCH

As an alternate form of this;

try dipping the wire in a baking-soda paste. The paste should stick to the wire. When emersed, the paste should begin to form bubbles and float the wire.

And a different form of this;
would be to place the wire in the water floating on a piece of tissue paper. As the paper soaks up water, it will become saturated and sink. This is a way of placing the wire "gently" on the surface if the cylinder is too narrow to get your hand in properly.

I'm not sure what a "cartesian diver" is but if it's anything like a density ball, that would work as well, but it would require that you place an air-tight covering over the cylinder. You could use an eyedropper. Fill the dropper with enough water so that it barely keeps the coil afloat. At this point, the eyedropper is in contact with the surface. However, if you stretch a bit of rubber over the opening, so that it forms an airtight seal, you can push down on the rubber and increase the pressure throughout the entire cylinder. This will force more water into the dropper, causing it to sink. Apply just enough pressure, and it will stop and hover in the water. This is how submarines work.

Of course, the hard part with subs is getting an airtight seal on your rubber sheet over the ocean!

7. Feb 5, 2004

### thundermist

Thanks!! (Daughter needs help)...

First: just wanted to say "many thanks" to those who have given suggestions.

Second: I was gently chastised by the group mentor for my posting on this website, so I apologize if I upset anyone by posting something inappropriate for this forum.

Third: However, since I've already opened this can of worms, and just in case anyone -does- want to provide answers, or just challenge themselves on this one, here's the catch:

This is an actual experiment that has to be carried out successfully to get a passing grade (so theoretical solutions won't work). In addition to the rules stated in my previous post, the cylinder can not have anything attached to it, it can't be moved, and the water level can not be altered.

The goal is to walk into a room with the experiment already set up and APPLY your solution according to the rules set by the teacher (high school).

My first thought was to make a Cartesian diver. However, since nothing can be attached to the cylinder, that creates a problem with being able to adjust the cylinder pressure, and therefore the rising/sinking of the diver.

This problem has already stumped several grad students and a PhD physics professor, so I hope you find this a good mental exercise.

Thanks again ~Mike~

8. Feb 5, 2004

### NateTG

Re: Thanks!! (Daughter needs help)...

You can always change the rules ad hoc to make things impossible e.g. no adding vinegar to the water or whatever.

You can always ask the teacher how it's possible to insert the copper without increasing the water level.

9. Feb 6, 2004

### Njorl

My cooling suggestion would work. It would not need to be helium, you could use liquid nitrogen, or possibly even the gaseous evaporant of liquid nitrogen. Anything far below water's freezing point would do it. You need to be able to locally freeze the water at the bottom of the cylinder. The ice will encapsulate the wire, while sticking to the edge of the cylinder.

I JUST THOUGHT OF A MUCH BETTER WAY!

Freeze the whole thing by any means. Get a hot air gun and melt the top preferentially. Eventually, a large chunk of ice will float off the bottom. Continue heating with the hot air gun until the ice chunk is small enough for the wire to be above the line.

Njorl

10. Feb 6, 2004

### ZapperZ

Staff Emeritus
OK, not sure if anyone before me has suggested this, or a variation of this, but what if you use the same principle involved in a Galileo thermometer?[1] Take, or make, one of the bubbles, and instead of using the weights, use the copper wire instead. You need a bubble that, in combination with the copper wire, will initially sink to the bottom of the cylinder when the water is at room temperature. [I am assuming that you are allowed to reach in and fish out the copper wire and attach it to the bubble when your daughter walks into the room to perform this]. To raise it, you will need some means to cool the cylinder+water. At the appropriate temperature, the bubble+copper wire will rise.

Unfortunately, if you want to do this, your daughter will need to do some preliminary calculations to estimate, based on the weight of the copper wire, the size of the bubbles that you need to initially make it sink, and then, with cooling, that it will rise without having to get to the point that you have to freeze the whole thing. You will need to know the variation of the density of water with temperature. I think this method might work, especially since your daughter was told that there is some space (7 inches?) from the top of the water level to the top of the cylinder - so there is room to immerse something like one of these bubbles without having the water overflow out of the cylinder.

I actually like challenges like this being given to students, even at the high school level. However, I dislike the fact that a passing grade is determined the way you described it. The students should not be judged solely on whether he/she managed to do this. Rather, they should be judged on the PROCESS, the methodology and rationality of how they come up with their solutions. I think in this case, how they got their solution is as important, if not more, than what the solution is. It is why we ask students in physics laboratories to write down what they did, what they observed, etc. in their lab reports, rather than just tell us what the results are.

Zz.

[1] See, for example, http://www.howstuffworks.com/question663.htm [Warning, may have pop-ups]

11. Feb 6, 2004

### NateTG

Zapper, thats a good idea -- here's a simplification -- if you use a 'cartesian diver' then it can be temperature controlled since PV=NRT applies in the bubble. The cartesian (unsealed) approach has the advantage that the rate of bouyancy change is much higher and construction is probably simpler.

Set up the diver so that it just sinks and then heat the cylinder from the bottom. (It's important to heat from the bottom so that the top is the coolest part.) The diver should *slowly* rise.

In order to do this efficiently, it would probably be a good idea to set up the diver with some sort of basket, and just place ballast in the basket (fishing weights are popular) to control the bouyancy.

Materials:

1 Plastic cup (bubble holder)
Wire (for construction)
1 Heater
assorted weights.

12. Feb 6, 2004

### thundermist

Like Zapper, I also believe such assignments are good mental exercises. They are great ways to make students "think outside of the box" and develop their deductive reasoning skills. However, the way this guy operates is to assign a project, give a strict set of guidelines, and then make it as difficult as possible for the students to pass (ex: 50 students must all come up with different methods of solving this project, no two methods can be the same, and he "guarantees" that there aren't any where near 50 solutions). So, most of his students will fail (much to his delight).

Regarding the assignment:

The "experiment" is already set up when the student enters the room. The wire is already at the bottom of the column of water. Nothing can touch the water and the air at the same time (no "fishing" for the wire). No water can be added or removed. No magetism can be use. The cylinder can not be moved. Nothing can be added to the water that will "significantly" change the composition of the water (guess who gets to define "significantly"?).

So, whatever solution the student comes up with, it's definitely a "hands-off" affair when it comes to lifting the copper wire. I think a Cartesian diver with a treble-hook might be able to grab the wire. If the "teacher" will allow the cylinder to be stoppered, pressure could be applied to cause the diver to sink.

With 50 students scurrying to be the first to come up with a answer, I'm afraid the "diver" will be used quickly. So, if anyone can think of a chemical or electrical solution, please let me know.

Thanks again, Mike

13. Feb 6, 2004

### NateTG

You could probably get away with just boiling the water.

Freezing the bottom as Njorl suggested would also work.

Re: The diver - the diver can be controlled by changing the temperature of the water. The hotter the water is, the more the diver will rise.

A gallilean thermometer (much more difficult) would have the opposite effect. Raising the temperature lowers the 'float' and lowering the temperature raises it.

If you disolve two reagents that form a gas in the water then you can also get a reaction. I think that the classic combination is Cream of Tartar and Baking Soda (or Baking Powder). You may have to heat the water to get the reaction to occur.

There's also the possibility of using an anchor&chain method to raise and lower something without hitting the surface. Making something like that effective requires knowing the dimensions of the cylinder.

My favorite solution is still to simply wipe the line off of the cylinder, and drop it on the floor.

14. Feb 6, 2004

### Cliff_J

Here's some low-brow ideas:

For the bubbles, a weighted container with a dissolvable membrane would sink to the bottom. Then once the membrane was gone a reaction would provide the bubbles. I'd use some pure sodium just for kicks! Or the cartride from the bottom of a Guinness beer can. But I'm sure more elegant alternatives would be better for the passing grade.

How about a simple remote controlled mini-submarine with a hook? That's what I would've built in HS.

Make the diver, adjust so it floats just below the surface with the additional weight of the wire. Design a system to hold weights used to sink the diver (that release at bottom from impact or dissolvable link) and then a hook to snare wire. Diver floats and carries wire but doesn't break surface.

Cliff

15. Feb 9, 2004

### Jwprox

I have an idea (probably won't take it though)

You should have her put some adhesive on a fish, then have it follow a peice of food outside of the jar to the copper wire, than drop the food on the surface so it swims up with the wire stuck to it. Or, assuming the wire is coiled, maybe you could tie a little hook around the fish because technically it wouldn't touch the air and the water.

Oh, and if the fish croaks, it's the physics teacher's fault.