27 February 2014

What Physics Teachers Talk About Out Of Class

Physics Teachers being Naughty

One of the nice things about being a member of the Western New York Physics Teachers Alliance is the email list. Here we can share ideas with colleagues  we may see only once every month or two at our regular meetings. The following is an example of one such exchange (names have been deleted to protect the guilty).

I've got a PVC cannon that can fire ping-pong balls/dog toys, paper tubes/rockets, and dry erase markers.  It's electrically triggered, but I'm still trying to figure out what the max pressure it can take and still electrically fire.  Individual components are good up to 90 psi, but around 70, the sprinkler doesn't actuate.  40 psi will send a dog toy at ludicrous speeds.  I also went out and got an air compressor at Harbor freight, so we can basically fire the thing every minute or two (depending).  Air compressor is loud as dickens, tho.  Actual muzzle velocity calculations/high speed forthcoming.

It can fire dog toys; can it fire toy-dogs (not the plushy kind)?

If you can get them to fit in the barrel.  And not wiggle around too much.

We could get a Doppler-shift demo out it, too. 


17 February 2014

5-g's? Let's not Exaggerate.

Watching the Olympics, I have seen a commercial that annoys me. It features a figure skater comparing what she goes through to other occupations. One is that she "experiences more g-forces than a fighter pilot". Let's examine that claim.

The math

The forces on the various parts of a spinning skater are due to rotation. However, we will look at the acceleration since that what g-forces really are. For a 5g-force, the relationship between rotation speed (RPM) and distance from rotation center (r) is

A graph might make this equation a little more meaningful.

So, we can see that a 5-g force can be obtained for a hand rotating 1 meter from the center at a little more that 60 rpm (or one rotation each second). But the head? Even with a skater's head held straight out at around 20 cm, 5-g's would only be experienced at 150 rpm (over 2 rotations per second). And if we look at a 9-g force which most fighter pilots experience occasionally (and for a lot more than a second), it would take 200 rpm. While the supposed world record for a figure skater spin is around 300 rpm, that skater held her head upright, so her nose might feel a 9-g force, her head would feel a much lower force.


What can we conclude? While it is possible that a figure skater can experience over 5-g's of force on some parts of her body, when was the last time you saw a skater do a rapid spin in competition? So, the ad is true, but very misleading.