music; sfx glass breaking, bar brawl, bottle breaking
TD: It’s very discriminating, yet very quick, and other than looking at the color and the thickness of the glass, it’s one of the quickest ways I can determine if the glass came all from the same source.
JM: Suppose your car is broken into
[sfx breaking glass]
and police find a suspect with glass fragments in his clothing.
[sfx bar brawl]
But the man claims he was involved in a bar fight, and that was the source of the glass. So, which is it? I’m Jim Metzner, and this is the Pulse of the Planet. Tamara Danner is a trace evidence analyst at the Wallie Howard Jr. Center for Forensic Sciences in Syracuse, NY. To reveal the true story, Danner turns to density.
TD: Density is mass over unit volume, so basically density is how much something weighs versus what size it is or how much space it takes up. There are two pieces of glass here that are the same color. They’re clear and colorless. And I weighed them earlier, and they weigh the same. However, if I put them in my density liquid and mix it up [mixing sounds] you can see that one piece of glass floats. It will float to the top of the liquid, and the other piece of glass will sink. The one that has the greater density is the piece that sinks to the bottom. So even though those two pieces of glass weigh the same, they’re made with different material, and it makes one’s density greater.
[sfx bar brawl and breaking bottle, up and under]
JM: So if inconsistent density corroborates the suspect’s bar brawl story
[sfx bottle breaking]
he’s off the hook.
Tamara Danner uses lab chemicals for her density tests, but you can do the same thing at home, using nothing more than water, oil, and honey. Visit kidsciencechallenge.com to find out how. Tamara is a participant in this year’s Kids’ Science Challenge, our free nationwide competition for 3rd to 6th graders, made possible by the National Science Foundation. I’m Jim Metzner.