My kids want to test the density of the NBN balls. I don’t know what difference the density could really make but I was impressed they wanted to do something scientific and so I don’t want to discourage them from doing the test. They want to immerse the balls in water to measure volume, but I’m concerned the balls might absorb water. So my question is:
Are the balls made out of closed cell or open cell foam?
I know they have a coating on the outside, but small exposures in the mold seams have exposed the bulk foam material.
The balls are made from 0.1g/cm^3 open cell polyurethane foam. The “skin” is a result of the mold release agent that we use.
If the balls are quickly submerged they will absorb a small amount of liquid. Also, these balls do float, so your students may have a difficult time submerging the balls without affecting the volume of displacement. To mitigate these effects, they could submerge the balls in rice or sand instead of water. Also they could encase the ball in a well sealed, airless, plastic bag.