Re: Can someone explain Eric Laithwaites Royal Institute lecture?

Hi Esteban,

I have to say your question is the most difficult one I've ever had to 
answer, and I'm thrilled about the level of complexity, coolness, and 
confusion that comes up when you consider gyroscopes.  I've got an answer 
that I'm willing to put down on paper and sign my name to, but I'm still 
only 99% sure that I'm right.  History may one day show that I and 
practically every other member of the scientific community were as wrong 
and as obstinate as the "flat Earth at the center of the universe 
surrounded by planets with orbits corresponding to heavenly ratios" 
people, but here's my answer:

The short answer is that a gyroscope will have the same weight whether 
it's spinning or not.  Prof. Eric Laithwaites may have done a bit of 
acting to try and make his point, but he must have been able to lift 50 
lb. in order to do his demonstration.  

HOWEVER, a rotating (and precessing) gyroscope will have a center of 
gravity at the support point, so it will definitely be EASIER to hold up 
a 50 lb. wheel at the end of a 3 foot rod if it is spinning.  There isn't 
a video or film of his presentation, but I bet it would look something 
like this:  He tries (and fails) to lift a motorcycle wheel stuck to the 
end of a 3 foot rod off the ground by lifting the other end of the rod. 
that makes sense.  If he then spun the wheel, then was able to lift the 
other end of the rod so that the motorcycle wheel precessed around him, 
then that's a pretty neat trick.  Without spinning, he not only has to 
hold up the 50 lb. wheel, he has to apply torque to the rod to keep the 
rod horizontal and the wheel of the ground.  With spinning, the rotation 
of the wheel is coupled (cross product) with the "precession" of the rod 
and stick in a circle around him, and this counteracts the torque.  The 
faster the wheel spins (and the more mass it has on the outermost edge) 
the more pronounced this effect will be.

As far as useful applications go, we all have to be very careful what we 
read on the internet and get from pseudo-scientists.  You can get objects 
to move a distance by messing with their center of balance, but at the 
end of the day, you can't get something for nothing.  In order for a 
space craft or any other transportation device to utilize "gyroscopic 
propulsion", you have to add energy.  You'll have to spin the gyros up to 
speed and slow them down, and apply a force to the gyros to move your 
craft.  In addition to the energy required to do these things, you'll 
have to deal with the law of conservation of momentum that says if you 
start spinning a heavy gyro in a certain direction, your ship will start 
spinning in the opposite direction.  (Why helecopters have tail rotors)  
The bottom line is that you have to apply a force externally to your ship 
in order to move it.  A gyro can change your center of gravity, but it 
won't change your momentum.  

I hope this helps! (And makes things a bit more clear.)
Jeff Yap
Mad Scientist

References:

NASA Common Errors 
in Propulsion physics

Gyroscopes.org (Great site 
for all things gyroscopic)

MadSci Network responses:
948303982.Ph
948896940.Ph
867347613.Ph

US Patent # 5860317

Buffalo State College Physics Department

PS: One last note.  Clarke's third law says, "Any sufficiently advanced 
technology is indistinguishable from magic."  Rocketships may one day be 
propelled by superconducting gyros being hit by RF radiation while 
spinning at relativistic speeds.  In this case, I will gladly retract 
this answer.