# Physics / Biomechanics / Weightlifting Question

1. Aug 1, 2012

### AnthonyLiardo

My questions is about the bar (not specifically) in this video:

I want to know from somebody who has a better mechanical perception than me why the bar has the effect of wobbling back and forth. I think it's because the center of the mass of the system (not including the person) is low and if the bar is a little forward or back it produces torque, and it seems that the lower the center of mass is, the more torque/angular acceleration is produced. Is this because r in $\tau$ = F $\cdot$ r is higher?

But if the center of mass is lower, doesn't that make the moment of inertia higher so $\alpha$ would be lower with the same torque? Or is it negligible because the bar weighs 5.4 lbs?

The reason I think so is because I have done this with a 45 lb. bar and it wasn't that unstable.

The biomechanics part of my speculation is to which muscles it really stabilizes. Because it's so wobbly, I think the muscles to push it over your face and over your stomach both have to be contracted so it stays in line. Which muscles would that be?

I really just want to discuss this, especially the moment of inertia, torque, angular acceleration part of the effect, with people so discuss with me please.

Last edited by a moderator: Sep 25, 2014
2. Aug 1, 2012

### Danger

I can't give any definitive answers. All that I can say is that what I saw resembled what was known as "over-controlled oscillation" back when I was flying. It sort of means that the initial response to an imbalance over-compensates and thus sets up an "inverse imbalance" (my term, and probably not proper). That then results in another over-compensation in the other direction, and so on until the thing rips itself to shreds. It's one reason that computers are necessary to fly modern fighter planes.

3. Aug 2, 2012

### haruspex

I think the problem is the mismatch between the high load and low inertia of the bar. As the bar wobbles, the suspended load is little affected. The longer the straps, the less affected the weights are by the wobble, so they offer little inertial resistance.
Imagine instead pushing a large heavy box across a floor. Now try doing it holding roller skates, pushing the box with the wheels. The skates can move up and down quite freely, and a small error in applying the force horizontally will lead to large vertical movements of the skates.

4. Aug 2, 2012

### sophiecentaur

This effect only happens when weights are dangling from the bar, I think. The poor lifter's muscles and control system are encountering a very unfamiliar situation because the masses are free to move horizontally over a significant distance despite the muscles detecting a large vertical weight force. Once there is any lateral movement in the arms (i.e. they stray from the vertical) the feedback signals are all wrong and the control goes unstable. It is something that I'm sure could be overcome with training / practice.

I have noticed a very similar thing when I first started using a cheap scaffold tower to paint the house. It would oscillate wildly when I was at the top as my legs desperately tried to counteract deflections - but in the wrong phase. Once my body learned to respond differently to the deflections (actually NOT respond) the problem went away and the oscillations were damped out.

5. Aug 2, 2012

### haruspex

It's not the masses, exactly, that move so easily; it's the line of action of the load.
I think this case is a bit tougher to handle. The lifter must respond: with his arms off vertical he has to deal with a huge moment. It's hard to combine fine motor control with large load bearing. It's more than just unfamiliarity; there's a very large gain in the feedback.