Making a superadjustable barbell My working knowledge of physics is the level of a 7th grader (if I'm lucky) so please excuse this probably totally stupid question, but my curiosity is greater than my shame:
Would it be possible to create a barbell that works without weight plates?
What I have in mind is something the shape of a barbell with some motor or whatever at the two ends that would be adjustable in weight without loading additional steelplates onto it.
(If the answer is "no": why?)
It would be awesome if anyone had thoughts on this.
 A: All ideas are worth exploring :-)  This question is mosly light engineering, but there is also some elementary physics regarding forces.
Actually, an answer to your question depends very much on what you
want to do with the barbell, on how much you want to move it around.
If it is simply for weight lifting, this requires only vertical
motion, an a bit of lateral (horizontal) motion to get some freedom of
mouvement for the weight lifter.
Then the plates can be replaced by a rope pulling the ends of the bar
towards the ground. The ropes can go through pulleys, so that the
force pulling on them can come from any direction.
Now, all you need is a controlled device that will adjust the force
pulling the rope independently of the motion of the bar.
One simple way is to to attach the other ends of the two ropes to a lever, far enough from
the axis so that it can be moved for about 2.5 meters, which is a bit
more than what is needed for the weight lifter lifting the barbell
from ground to vertical position with arms extended (but you do not
wnat him blocked as he is lifting.  Then, you can use a motor to move a
weight along the arm of the lever so that the pull on the rope can be
adjusted to whatever force is required.
This of course takes much room. You can reduce the size of the lever
by using blocks and tackles, so that the motion of the ropes can be
increased, But this requires moving larger weights on the lever.
There are probably other devices that can provide an adjustable
constant force on one end of a rope which allowing some motion on the
free end of the rope.
I leave it to you to study the physics of the system.
If you were thinking of changing the lifting force required by moving things around on
an independent barbell device, the answer is no. You are simply lifting the
weight of whatever constitute the device you lift, and that does not
change, assuming you do not change its mass (by adding or removing parts) and you do not change your location (the same mass can produce a different weight when gravity changes, for example on the moon).
Even relativity, very popular on this site, will not help you.
Though special relativity has the reputation of causing mass to increase with speed,
it will not do that for you, because lifting the mass implies that you
travel with it, and that the planet creating the gravity for the
weights also travel with it. So that, as far as you (or the weight lifter) are concerned, there is no speed, no motion.
Thinking twice about it, relativity can help you, if you are willing to take the risk and can afford the energy and much else (but check first with a relativity engineer). The trick would be to rotate the plates at relativistic speed. All you need is quasi infinitely strong material (else the plates will explode under centrigugal forces) and perfect frictionless bearings that will not transfer torque to the holding bar. Also you have to use an external energy source to rotate the plate, else the total lifted would not change (mass-energy equivalence). I probably forgot a few thousand things, but solving the above should already keep you busy for some time. One last piece of advice: avoid dropping this barbell on the ground, as I am not sure how much of the planet would survive.
Also, do not try to turn around while lifting to show how strong you are: it will simply not work (and if anything did happen, you might not like it).
Real relativity specialists (which I am not in the least) may be able to say more on this :)
