Re: Pendulum mechanics

Michael Lorrey (
Fri, 18 Jul 1997 22:30:32 -0400

Lee Daniel Crocker wrote:
> > How can a little kid (or an adult for that matter) sit on a perfectly
> > still playground swing, begin rocking back and forth, and within a
> > minute be wizzing back and forth to a pretty good height, without ever
> > touching the ground or other still grounded object?
> Body movements can alter one's center of gravity significantly. Just
> as a high jumper can clear a bar without ever having his center of
> gravity above it, the child on the swing can move his legs in or out,
> and bend at the waist, to move his center of gravity just a touch
> higher on each swing than the height attributable to the energy of
> the previous reciprocal swing. On the downstroke, he repositions his
> body again to prepare for the maneuver on the other side, which uses
> the energy of his muscles without depleting any of the gravitational
> potential energy he has gained. The amount of extra energy put into
> the system from his own muscles is enough to escalate each swing.

But what is the energy put into? If true action/reaction holds, the
rider moving its center of gravity higher will only reduce the momentum
at that point in the swing, so it will not move any higher. If true
action/reaction holds, as it is popularly understood, the rider should
not be able to gain any momentum on each swing without causing a

			Michael Lorrey
------------------------------------------------------------		Inventor of the Lorrey Drive

Mikey's Animatronic Factory My Own Nuclear Espionage Agency (MONEA) MIKEYMAS(tm): The New Internet Holiday Transhumans of New Hampshire (>HNH) ------------------------------------------------------------ #!/usr/local/bin/perl-0777---export-a-crypto-system-sig-RC4-3-lines-PERL @k=unpack('C*',pack('H*',shift));for(@t=@s=0..255){$y=($k[$_%@k]+$s[$x=$_ ]+$y)%256;&S}$x=$y=0;for(unpack('C*',<>)){$x++;$y=($s[$x%=256]+$y)%256; &S;print pack(C,$_^=$s[($s[$x]+$s[$y])%256])}sub S{@s[$x,$y]=@s[$y,$x]}