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Deejay_Spike
11-11-2002, 11:15 AM
MOMENTUM : THE TRUTH REVEALED

INTRODUCTION : a quote

*Disclaimer* The following quote is intended to explain the misconceptions that subsist around momentum. It is intended for educational purposes, and not to bash this or this training method.

“One of the goals of superslow is to reduce offloading which is created by the momentum of the weight. On a squat for example, when one is ‘driving’ the bar up, the upwards force will offload the weight to some exent depending on speed, disctance, etc.”

PART I : a definition

Many people talk about momentum, but what IS momentum?

Momentum is a corner stone concept in Physics. It is a conserved quantity. That is, within a closed system of interacting objects, the total momentum of that system does not change value. This allows one to calculate and predict the outcomes when objects bounce into one another. Or, by knowing the outcome of a collision, one can reason what was the initial state of the system.

Momentum is mass times velocity. When an object is moving, it has a non-zero momentum. If an object is standing still, then its momentum is zero.

Momentum is a vector. That means, of course, that momentum is a quantity that has a magnitude, or size, and a direction. That direction can be positive (to the right) or negative (to the left) when in a one dimensional situation. omentum problems can become more complicated, however. Momentum problems can occur in in two and three dimensions. Under these conditions, say in a two dimensional problem, one would state a momentum using language such as '3.0 kg-m/s in a direction of 50 degrees North of West.'

! ATTENTION !
Momentum is not velocity. Sometimes the concept of momentum is confused with the concept of velocity. Do not do this. Momentum is related to velocity. In fact, they both have the same direction. That is, if an object has a velocity that is aimed toward the right, then its momentum will also be directed to the right. However, momentum is made up of both mass and velocity. One must take the mass and multiply it by the velocity to get the momentum.

Momentum is directly proportional to velocity. If the mass is kept constant, then the momentum of an object is directly proportional to its velocity. That is, if one were to double the velocity of an object, then the momentum of that object would also double. And, if one were to change the velocity of an object by a factor of 1/4, then the momentum of that object would also change by a factor of 1/4.

Momentum is directly proportional to mass. If the velocity is kept constant, then the momentum of an object is directly proportional to its mass. That is, if one were to triple the mass of an object, then the momentum of that object would also triple. And, if one were to change the mass of an object by a factor of 1/2, then the momentum of that object would also change by a factor of 1/2.

PART II : correcting the quote

According to Newton's 2nd Law, an increase in upward acceleration will increase and not "offload" the force exerted on the lifter. The only way to offload is to accelerate downwards with a load, not to slow it down while going upwards.

The only way to totally eliminate production of momentum is to do isometric training. Incidentally, load or force do not change with speed of repetition, but with acceleration, no matter what speed you are moving at.

This quote also implies you would be closer to your ‘true’ max with a very slow movement, because your muscles are moving the weight with little or no assistance from the speed of your lift.

If we follow this reasoning, then your 'true' max will be produced when there is no momentum involved - and, as I commented earlier, this happens under isometric or static conditions. In any strength contest or laboratory test, your 'true' max is the maximum load that you can lift irrespective of speed, acceleration or pattern of movement.

PART III : momentum and exercise

Many people insist that momentum always makes it easier to complete a movement. While this may be true with lighter loads, as the load increases, the ability to accelerate the load decreases and becomes very small when you approach your maximum load in any exercise. With a circamaximal or maximal lift, one does not have to deliberately have to try to slow the movement down, because that is an inevitable consequence of increasing the load. If only we could use momentum when we are doing any maximal lift!

Any experienced bodybuilder and competitive lifter tries to accelerate the load against gravity because this ensures that it will be possible to complete the lift successfully - this attempt to accelerate a heavy load usually does not create some exceptional "rocket-propelled" change in force and velocity, but it provides just enough sustained or increased drive to allow the lift to be completed.

It is therefore very naïve to suggest that one uses momentum to complete any maximal lift. Because if momentum were playing a significant role, then we could stop pushing well before the end of a very heavy bench press or squat and the momentum would simply carry the load to the end. Like I sais, this certainly is possible with light loads, but not with the sort of heavy loads which really increase strength or test you at the limits of your performance.

PART IV : an example

Many people who have the wrong idea of momentum, also seem to bash Olympic weightlifting, since they state that the Olympic lifts and their variations rely primarily on momentum and not production of 'real' muscle force. This reveals a very clear ignorance of weightlifting biomechanics and I would suggest that they study some of the scientific
analyses of the Olympic style lifts in a little detail.

It’s like the rapid pull in the snatch, the clean, power clean, power snatch etc creates so much momentum that the lifter at some later stage of the lift just stops pulling, simply stands around for a short while and allows momentum to magically carry a maximal load to a greater height where the lifter just catches it in the right position. What they are forgetting is that momentum involving a very heavy load will only allow the bar to persist in upward motion for a very small distance, and that the lifter successfully completes the lift by pulling himself under the bar before it beats hir or her to the platform.

The same people also try to create the impression that explosive and ballistic movements are dangerous and, on this basis, conclude that no athletes should ever use the Olympic style lifts or their variations for conditioning. Again, this shows little familiarity with sports biomechanics, because research shows that the common sporting actions of running, jumping, striking, hitting and falling can easily create far greater forces and impulses than those produced in the weightlifting movements. If we were to accept their philosophy of movement safety and effectiveness, we logically should avoid anything like running, jumping and so forth.

When argueing on this very subjects some comments like these arose : some say that “the only way Olympic style lifters can lift the masses that they do is because the proportion of the total load they experience as the terminal portion of the lift is approached is low due to the relative high velocity of the bar that was accelerated earlier in the lift. In this case, off-loading definitely occurs, and is vital to the completion of the lift.”

People are clearly mixin some things up. The weight of the bar, not its mass, at any instant does not depend at all on its velocity, but its acceleration. Even if the acceleration drops to zero, then the weight of the bar is still what the bar weighed while it was at rest on the platform. If the loaded bar weighed 400lbs on the platform and the lifter caught it when it was no longer accelerating up or down due to its movement, then it would still weigh 400lbs when it was caught successfully on the lifter's chest during the clean.

Moreover, while the lifter is pulling the bar during the clean, its weight varies between its resting weight on the platform and as much as 1.8 times that weight at the commencement of the "second pull". Interestingly, the lightest the bar becomes when it is off the platform is during the so-called "double knee-bend" (or scoop), not near the termination of the lift, as is being stated.

Also, what is referred to as high velocity is not even a fraction of the velocity of a limb when you are running, throwin or striking. The maximal bar velocity produced by top level weightlifters is somewhere around 2m/s, which is less than 5mph or 8kph. Offloading definitely doesn’t occur when the bar is accelerating upwards. Newton’s second law reminds us that an increase in upward acceleration will increase and not "offload" the force on the bar and on the platform on which the lifter is standing.

CONCLUSION

All I would like to say is that this article was to make the myth around momentum disappear. As they say “Knowledge is power,” but “little knowledge is dangerous.”

Retardo-pex
11-13-2002, 02:55 PM
finally.

John Henry
12-05-2002, 08:43 PM
We need more of this. Good job.