Physics experiments underwater?

[krikdiver]

I'm a science teacher on Kwajalein Island, of the Republic of Marshall Islands. I teach 7th grade Earth Science, 10th chemistry, 11th physics, and 12th zoology. The reason I've joined scubaboard.com is to get ideas from all of you for some underwater activities for my students.

A surprisingly small number of students are scuba certified, but this year, all three of my physics students (super small class!) have their open water ceritifcations. One of my colleagues is a dive master, and I have my Advanced Cert, so I'd really like to take the students underwater to perform some experiments.

What I am asking from the scuba forum is this: Any and ALL ideas for underwater physics experiments! Some of our units include: Linear/Projectile motion (just finished both of those), Newton's Laws, circular motion and rotational mechanics, properties of matter (I already have lots of ideas for the gas laws!), sound, light, electricity, and magnetism.

We don't really have any limitations. I can take these girls (with the assistance of my divemaster colleague) to go night diving any time, we can go in the pool, we can go into big sandy expanses in the lagoon... but I'm figuring this is my only chance to take a physics class underwater.

Thank you for your ideas!!

 

[TSandM]

You could do some kind of experiment involving heat transfer -- take several packages down, one perhaps in a ziplock with air in it, one in a ziplock with the zipper just barely open (so it fills with water but there is little circulation) and one with the zipper mostly open. Keep the items in the water a given amount of time and record the temperature reached.

 

[Doppler]

Simple one would be to show the difference between mass and weight. A spring scale should work underwater and on land but don't attempt with a digital one! LOL

Along similar lines, you can demonstrate inertia but getting two objects, one with very little mass the other with much more, get both 'neutral' and measure the work needed to get each moving.

And you could demonstrate that gas has mass by showing buoyancy shift in a small volume cylinder after dumping or breathing its contents.

Finally, how about light? Demonstrate properties of light energy as depth increases.


Good Luck! Sounds like a worthwhile project.

 

[Foxfish]

Take a length of wood underwater and put weights at different locations until it starts to sink then find the point where the wood needs to be supported for equilibrium. Don't recall if they learn about buoyancy at this level but it can be modeled as a force at the centre of the beam equal to the volume of water displaced by the beam. Use a spring scale to check the magnitude of the reaction. Conversely, hold the beam at one end and then change the position of the weight to again find the equilibrium position. Calculate the result in class first and then check to see how it compares underwater.

Analyse the forces exerted by a lift bag and then check results underwater. For equilibrium, the mass to be lifted should be equal to the mass of the water displaced by the air.

Check PV=mRT relationship for gases with scuba tank full of air. Leave in room for a period. Measure room temperature and tank pressure then calculate the mass of air in the tank. Leave out in sun and see how much pressure increases. Estimate average temperature of gas in tank. Take tank underwater (not as diver air supply) and note reduction in tank pressure. Again estimate the temperature of air in the tank.

 

[Akimbo]

Are any of your students Navy brats? The Navy has a chamber on the island and you might be able to get a chamber ride for the class if you ask the right people. Diving lockers tend to be a little less uptight in remote locations. No cost for asking. A chamber ride is a lifetime experience for most people.

 

[dfx]

Loss of colour at depth... maybe the different rates at which different colours get scattered by the water. You could take two waterproof laser pointers of different colours (red, blue/green?) and measure how far the beam travels before it disappears.

Different refractive properties and air vs. water also come to mind. Maybe create an optical lens out of air?

 

[3D diver]

Demonstrate the water pressure by watching an air filled balloon compress with depth and/or fill a balloon at depth and watch it pop on the way up. (why we don't hold our breath, eh?)

I wonder if you can demonstrate gas absorption into liquids this way: take a container with water and air in it under water, keep it there for a while then see if bubbles form when it's returned to the surface. (the bends).

 

[Basking Ridge Diver]

Measure sound traveling through water versus air. Talk about the density of air vs density of water.
Try making a pendulum and see what the difference is on the surface (air) and at various depths - does fresh water or salt water make a difference in the period?
Displacement verse volume measurement are interesting to me.

 

[Doc]

Start at the surface. Play spearfish games with a pole and a submerged rock or prepared target. With their heritage, this might be a well known phenomena, but explaining it is the question.

Experiment with soda pop underwater. Does it fizz at depth?

Aerosol cheese stops squirting out at 25' or so. Have them explain why... And do the math to determine what psi the contents are under when newly opened.

Dye some fresh water dark with food dye and dive with it in a soda pop bottle, Does it float? Open it u/w... Try pouring it out (or up).

Tie a styrofoam cup in a mesh bag with a weight and 200' of line. Try sending it down from a boat- see how it crushes at different depths.

Crack an egg underwater.

Take a balloon down, watch it contract. Inflate one at depth, tie it off and release it.

Fill a a pop bottle with air at depth, screw on the top. Others can observe it as it surfaces. Did the cap blow off?

investigate why a simple glass magnifier works u/w, but a plastic version is marginal.

Try any mechanical assembly that involves applying torque- something like astronauts might practice at the Neutral Buoyancy Lab. Even something as simple as driving a screw or setting/removing a nail.

Practice all the experiment steps above water, try it on your own u/w. One huge thing you will learn is that while working u/w, everything pretty well goes wrong.

Make sure your young scientists are overweighted enough to allow them to go "kerplunk" on the sandy bottom. Less thrashing around, the better. They may well be using air at a substantially faster rate than their dive history might predict.

Clean up the mess, of course.

 

[Matt S.]

I would think that the physics of decompression would be interesting to scuba certified physics students. Maybe you can do some kind of experiment with gas-loaded liquids and offgassing.

IMHO the best lessons will be actual experiments, where you compare predictions and observations, and not just demos.

Lucky kids.