Date: Thu Dec 20 16:16:16 2007

Author: John Welch

Subject: Re: two questions


So you're hypothesizing that if I put 3 or 4 rods under the board
instead of 2 that the effect will disappear?

Bill Norwood wrote:

Two items:

1. Language: Have a person lie down...

2. How about: You'll see that sufficient body mass shifting occurs atop the
board to flex the board sufficiently, so that its length between the two
supporting cylinders varies, so that one or both of the cylinders will roll
slightly, and you will see that this variation is synchronous with the
rhythm of what must be the person's heartbeat.

Bill Norwood
U of MD, College Park

-----Original Message-----
From: [] On
Behalf Of John Welch
Sent: Thursday, December 20, 2007 3:45 PM
Subject: Re: [tap-l] two questions

I just tried this with the students studying for their finals in our
physics study center - it worked great! thanks for the idea.

Even more interesting: Setup a ballistocardiogram. Have a person lay
on a board that is sitting on some thin round metal rods. Affix a
mirror to one of the rods. Shine a laser on the mirror so that it
deflects to the ceiling or some distant wall. You'll see that the
person's body moves up and down in response to the beating of his or
her heart.


John Welch

Cabrillo College Physics Dept.