Another idea

Well, I tested the elasticity experiment today and it didn't go well - apparently the vibration of the rulers is too slow for a 400 Hz tester. It just could not pick up the vibration! (But I tried yelling at the apparatus and that worked extremely well. But human tests require 30 something people, and it's such as hassle to get people to agree to do it, sign the permission forms and not turning up).

So the new idea is that I would test the rate iron rusts in different PH solutions. I can use vinegar for acid and dishwash for base - and use water to water them down to different PH. I'll buy a new pack of nails, so that they are the same (a controlled variable) and put them in solutions of the same volume in closed similar containers (to prevent water from evaporating). And, I found out that rust is less dense than iron, so I would just measure how much the water rises up in the container (that's why they have to be the same shape and size) - the mass of the things in the containers don't change. Seems pretty nice. But I still have to find the difference in densities - if they are just 1 gram apart in a cubic metre then my experiment can't work!