Tuesday, November 04, 2014

Groovy… but I won't show it in class

Except as a springboard to a discussion of "What did they do wrong this time?"

But it is groovy. The world's largest vacuum chamber is used to perform the a variation of age-old physics classic, "penny and feather" free fall experiment.

Brian Cox visits the world's biggest vacuum chamber - Human Universe: Episode 4 Preview - BBC Two

Here's a video clip that I do show in class: A hammer and a feather dropped on the Moon.

Feather & Hammer Drop on Moon

Brian Cox is many kinds of wonderful, but showing free fall in a vacuum chamber using high-speed (slow motion) video, alone, acts to deceive.

A common misconception among physics learners is that gravitational acceleration depends on atmospheric pressure. Things float in space because there's no air in space. There's no reason to think g in the giant vacuum chamber is 9.8 m/s2. All video of free fall in the evacuated chamber is artificially slowed. We never see the vacuum chamber free fall in real time.

The lesson could be interpreted that things fall more slowly in a vacuum. On Earth as it is in Heaven. Or the Moon, at least.


Dean Baird said...

There is a moment during the Einstein story that appears to show the drop at something approaching real time. But one could easily interpret that half second of video as having been artificially sped up.

Lee Trampleasure said...

Dean, you raise an excellent point. When I first saw the video, I assumed the first shot would be real time, followed by slow motion of the landing. Maybe if enough of us educators complain they will release a re-editted version :-)