Letting Gravity Take the Pole- a discussion

[jcoover]

This is an area that I've been very interested in lately. I've been watching Tim Mack's 6.01 jump basically on repeat for the last week. He really does an excellent job letting the pole tip fall correctly and letting the pole tip "pull" him into the box, helping him to accelerate through the last 6 steps. The thing that I notice with Tim's run is the passive left hand. Much like Bubka, Mack just sort of rests the pole in his left hand. Now most vaulters do this, but I really feel like Tim and Bubka were able to shift the weight of the pole better than most into the right hand during the run, allowing for a very efficient fulcrum point on the left hand and therefore a successful "pole drop". The hips never tuck, and the action of the pole tip raises the chest allowing for a high takeoff with a good leading chest.

[jcoover]

PS I agree with Barto on the pole drop vs. speed discussion. Rest a pole on its end in your palm with the tip facing straight up, not using your left hand for support at all. As the pole begins to fall, in order to not let it fall to the ground, you will run towards the direction that it is falling to slow the velocity of the pole tip and eventually bring the pole back to vertical balance. I'm sure all of the kids at your high school do it all the time. This is a perfect demonstration of how running velocity slows the speed of the pole drop.

[kmonty51]

Wouldn't the speed of the pole drop be effected by the position of the top hand? If the top hand has flipped up and under the top of the pole while running, the top of the pole is, in effect, being driven up, forcing the tip down faster. Balancing a pole in your palm is more about trying to keep the palm under the pole at all times. So...initially speed of pole drop would be slowed by the jumper's approach speed, but pole drop speed would accelerate once the top hand "shift" were made. If the tip has dropped below shoulder height (approx.), it's going faster, I would think.

In defense of the HS physics teacher (former HS teacher here), some students fail to retain knowledge beyond the most recent big test (if that!). Because someone doesn't remember/understand a concept doesn't mean it was not taught correctly. Students have a responsibility in the learning process, as well. I know Barto's comment was probably meant in a humorous vein, but I get tired of hearing every screw-up blamed on poor teaching.

[ArcadiaPV]

Since you were all so concerned about my Physics teacher, he happens to be my roommate and the one that created the sim pole drop program. I'll let him handle this issue He is an aerospace engineer at UCLA, this might get technical

Okay, there's some confusion here in regards to reference frames. In the situation we're working with, I'm assuming that the pole vaulters are running as fast as they can. If this is the case then there is an initial burst of acceleration to reach their top speed. After reaching their top speed they will no longer be accelerating because, well, they've reached their top speed. If anything they might slow down. Regardless, once they've reach their top speed any speed changes afterwards are most likely negligible. Also worth noting is that their top speed is reached well before the pole has started dropping (at least in the technique we're working with).

What this all boils down to is that when analyzing the pole fall we can safely make the assumption that the pole vaulter is moving at a constant velocity. When dealing with the pole fall we take a reference frame relative to the runner. If the runner is moving at a constant velocity then this reference frame is an inertial reference frame so there are no external forces acting on the pole (other than gravity). In other words, we can treat the pole fall as if it was just falling from rest.

The situation you guys describe is with an accelerated reference frame (the pole vaulter speeding up). If the pole vaulter was accelerating while dropping the pole then the pole will indeed fall slower or even fall behind the pole vaulter. However, this only occurs with an accelerated reference frame (i.e. when the pole vaulter is speeding up). That acceleration amounts to a force forward at where the pole vaulter is holding the pole. This is thanks to Newton's Second Law, F=ma. The force is a function of acceleration so if the pole vaulter is speeding up then he/she generates an acceleration that results in a force. However, if the runner is going at constant velocity then there is no acceleration and thus no force.

In regards to air resistance, it's negligible (unless maybe if there is a strong head wind, though this affects the speed of the runner more than the pole fall). The speed at which everything is occurring is slow enough that all of the air flow around the pole can be considered laminar (smooth). In fact, its so slow that this can most likely be described using velocity potential theory. In velocity potential theory there is no net drag on an object in potential flow (which is what the flow around a pole moving that slow is).

To push my point home though, I'll calculate the Reynold's number. The Reynold's number is a dimensionless constant that describes how laminar or turbulent a flow is. If the flow is laminar then any air resistance can be ignored. Flow is considered laminar if the Reynold's number for the flow is less than about 2000. The Reynold's number can be calculated from the following:

Reynold's Number = (density)*(flow speed)*(diameter)/(dynamic viscosity)

For air, (density = 1.2 kg/m^3), (dynamic viscosity = 18.27 Pa-s) [Pascal seconds = N*s/m^2]
The pole is about 3 inches thick so (diameter = 3 in)
The runners were running at about 7.6 m/s so (flow speed = 7.6 m/s)
Calculating this we get the following Reynold's number:

Reynold's Number = (1.2 kg/m^3)*(7.6 m/s)*(3in)/(18.27 Pa*s) = 0.038

Obviously this is a lot less than 2000 which means that the flow is really laminar. Therefore air resistance on the pole is negligible. Air resistance on the runner is not though.

So yea, I hope that clears it up. I posted some sources you guys can take a look at below. Thanks for reading!

http://id.mind.net/~zona/mstm/physics/m ... Frame.html
http://en.wikipedia.org/wiki/Reynolds_number
http://www.media.mit.edu/physics/pedago ... acles.html
http://en.wikipedia.org/wiki/Viscosity
http://www.diracdelta.co.uk/science/sou ... ource.html

[Barto]

Okay, now we are getting somewhere.

Your physics is not wrong, but your concept of pole vaulting is.

Pole vaulters ARE accelerating while the pole is dropping! The technique you describe of accelerating to top speed and then trying to maintain it while dropping the pole will not lead to a very competitive performances and will need to be unlearned if the athlete is going to progress past high school.

[achtungpv]

Okay, now we are getting somewhere.

Your physics is not wrong, but your concept of pole vaulting is.

Pole vaulters ARE accelerating while the pole is dropping! The technique you describe of accelerating to top speed and then trying to maintain it while dropping the pole will not lead to a very competitive performances and will need to be unlearned if the athlete is going to progress past high school.

Had a conversation with dj once in Reno back in '97 where we discussed would it be "better" to hit a wall traveling at 60MPH for 100m or or accelerate to 60MPH at the instant of contact (takeoff). We both thought that the latter is preferred since you are theoretically increasing your input energy through the wall (takeoff). I think this is what Petrov refers to when he says most Americans come out of the back and get up to full speed too quickly. What's the benefit of running 18 steps if you're at top speed at 10? You might as well jump from 10.

[dougb]

You don’t have to support the poles weight

It seems to me a lucky coincidence that we have two threads going concerning runway speed.

One, pushing the pole along the runway as you run so you don’t have to support the poles weight.
Two, dropping the pole as you run so you don’t have to support the poles weight.

I have read on this board that Sergy Bubka/ Petrov first introduced the method of starting the run with the pole vertical and caused a considerable stir by doing so. Until that time vaulters ( myself included ) ran with the pole horizontal to the ground. I postulate that the advantage Sergy gained by the new method was not in the last few steps approaching the box, but in a overall increase in speed because he was timing the drop so that the pole was essentially weightless. After all, if you are going faster at your mid mark then, chances are ,you will be going faster at takeoff.

This does not mean that the left arm is doing nothing. Adjustments have to be made for everything that will affect the drop, and the left arm is in position to make those adjustments without materially lifting the pole.

If this is what everyone else is saying then I apologize for repeating.

doug

[ArcadiaPV]

First I would like to correct a typo in my last post. The diameter of the pole is not 3 inches

Barto- with this technique I usually have my vaulters on a 8 stride run. Meaning by the time they hit their drop mark, ~50 feet they are close to their top speed, but do continue to accelerate. The amount they are accelerating is not enough to affect the fall speed of the pole though.

I'm glad I got the response I did. This is an issue I've always wanted others opinion on. I learned the technique from Brian Yokoyama and Gio Lanaro. Both of them have had tremendous success with it. I think the most dramatic example was Tim Mack. He constantly had a problem being under and low. Once he started working on this form of the pole drop his take-off improved dramatically. This wasn't the only factor that contributed to his success but I know it was a big factor.

[Pogo Stick]

Okay, now we are getting somewhere.

Your physics is not wrong, but your concept of pole vaulting is.

Pole vaulters ARE accelerating while the pole is dropping! The technique you describe of accelerating to top speed and then trying to maintain it while dropping the pole will not lead to a very competitive performances and will need to be unlearned if the athlete is going to progress past high school.

I agree with Barto, physic is good, but something is missing. During dropping the pole, center of mass of pole itself, as well as whole vaulter-pole system, is moving ahead. Effective weight of vaulter itself is decreasing (of course if vaulter is keeping shoulder and torso in same position - this is critical). If mass (weight) is decreasing, less power is required to preserve same velocity. Or even better - the same amount of power will produce more speed (acceleration/force/energy). The result is like someone is pushing you ahead or remove couple pounds from you.

--
PoGo

[Tim McMichael]

This is something I have found to be true time and again. Physicists are great at what they do, but they assume the dynamics of the vault are simple, and they make all sorts of mistakes as a result. The problem with this is that the science lends authority to their conclusions, and this can lead coaches and athletes down the wrong road. This is nothing against physics or physicists. We need more of that sort of study. I just wish that the conceptual starting point could be more accurate more often.

[Tim McMichael]

Okay, now we are getting somewhere.

Your physics is not wrong, but your concept of pole vaulting is.

Pole vaulters ARE accelerating while the pole is dropping! The technique you describe of accelerating to top speed and then trying to maintain it while dropping the pole will not lead to a very competitive performances and will need to be unlearned if the athlete is going to progress past high school.

I agree with Barto, physic is good, but something is missing. During dropping the pole, center of mass of pole itself, as well as whole vaulter-pole system, is moving ahead. Effective weight of vaulter itself is decreasing (of course if vaulter is keeping shoulder and torso in same position - this is critical). If mass (weight) is decreasing, less power is required to preserve same velocity. Or even better - the same amount of power will produce more speed (acceleration/force/energy). The result is like someone is pushing you ahead or remove couple pounds from you.

--
PoGo

Great vaulters have a center of gravity that is rising as they come into the takeoff. Most vaulters sink. The difference can be as much as a foot of height with the same power. (As usual, I have no numbers, just personal experience.)