B Can a skateboarder ride faster by holding a water bottle?

[Devin-M]

I tried to lift my toe off the ground as far back as possible with front foot in the same location with and without bottle, I found I could lift toe off ground 10 inches further back while holding the bottle outstretched.

 

[Drakkith]

I tried to lift my toe off the ground as far back as possible with front foot in the same location with and without bottle, I found I could lift toe off ground 10 inches further back while holding the bottle outstretched.

I just performed a similar experiment and could extend my leg all the way backwards, toe just off of the ground, until the leg holding me up was so bent that I could no longer get closer to the ground to extend my leg any farther. I used no water bottle or other counterweight, just my own body.

 

[Devin-M]

Start with rear toe resting on ground and measure how far back you can possibly lift it, then hold out the water bottle and see if you can lift toe off ground from a farther back position. Essentially the question is can you stretch your leg farther back touching the ground, and still lift the toe off the ground when counterbalanced by the water bottle.

 

[Drakkith]

Start with rear toe resting on ground and measure how far back you can possibly lift it, then hold out the water bottle and see if you can lift toe off ground from a farther back position. Essentially the question is can you stretch your leg farther back touching the ground, and still lift the toe off the ground when counterbalanced by the water bottle.

I can extend my leg as far back as possible, toe on the ground, and then lift it without any counterweight required. Can you not?

 

[Devin-M]

I can extend my leg as far back as possible, toe on the ground, and then lift it without any counterweight required. Can you not?

I can without counterweight, but I can stretch it back 10 inches further and lift with the water counterbalance.

 

[Drakkith]

I can without counterweight, but I can stretch it back 10 inches further and lift with the water counterbalance.

No no, I meant that I can stretch my leg all the way back, as far back as it is physically possible to get, and still lift my toe off the ground without using a counterweight.

 

[russ_watters]

If the stroke is longer it’s conceivably force multiplied by longer distance per kick.

The distance, as @Drakkith and @DaveC426913 are trying to tell you, is the length of your leg either way. Your leg extends from your hip to your toe, period (if you are flexible enough to bend that far). Moving your hips backwards 10" just moves the 0 point backwards 10", it doesn't change how long your leg is.

As discussed previously though, if you swing your arms forward while kicking backwards, that accelerates your COM faster by moving it forward.

 

[russ_watters]

I just took a spin, here’s what I found:

I use the water bottle arm forward in 2 situations:

-already going fast on flat ground, 2 feet on board, slowed down a bit from wind drag, lift right arm rigidly forward with bottle, then take left foot off board, 3 kicks only right foot on board, foot back on board, lower arm back to side, trying to regain maximum possible flat ground speed. It “feels like” the bottle helps me reset my stroke in less time.

-pushing up hill, the arm forward with bottle feels even more useful in this setting than accelerating on flat ground.

"feels like" is far too imprecise/qualitative to be of value. I think what you should do is mark off a 100m course (exact distance not critical) and time yourself in a few sprints in each configuration.

 

[Devin-M]

No no, I meant that I can stretch my leg all the way back, as far back as it is physically possible to get, and still lift my toe off the ground without using a counterweight.

Was your front leg's knee straight or bent? When I got the longer rear leg reach holding the water bottle forward, my front knee was bent more and so body position lower while having the extended reach and lifting rear leg toe off ground.

 

[A.T.]

I tried to lift my toe off the ground as far back as possible with front foot in the same location with and without bottle, I found I could lift toe off ground 10 inches further back while holding the bottle outstretched.

That is a semi static situation where the net torque around your CoM must be close to zero all the time.

In a highly dynamic movement this is not longer the case. The only requirement is that the net angular impulse over a cycle is zero. In other words, the average net torque over a cycle must be zero, not the instantaneous net torque at any time point.

 

[Drakkith]

Was your front leg's knee straight or bent? When I got the longer rear leg reach holding the water bottle forward, my front knee was bent more and so body position lower while having the extended reach and lifting rear leg toe off ground.

It was bent. This is exactly what I did.

 

[A.T.]

If the stroke is longer it’s conceivably force multiplied by longer distance per kick.

Possibly. But it could also be that the lower stroke frequency outweighs any gain from the longer stroke.

Finding the optimal stroke length is the goal in many sports, and its rarely at the physiological maximum.

 

[Devin-M]

It was bent. This is exactly what I did.

If you're almost doing the splits, I don't see how you could lift rear toe in this position? I certainly can't lift rear toe with leg as far back as I can make it go.

 

[A.T.]

It “feels like” the bottle helps me reset my stroke in less time.

Reseting the stroke is a different balance situation than the acceleration phase with one foot on the ground, that you were initially theorizing about.

During reset you have to accelerate your leg forward while having a small base of support. Here, having weights in stretched out arms increases their moment of inertia, and thus improves their capability to counter small perturbations of balance.

 

[Drakkith]

If you're almost doing the splits, I don't see how you could lift rear toe in this position? I certainly can't lift rear toe with leg as far back as I can make it go.

I'm not doing the splits, I'm as far back as I can physically go with one leg under me.

 

[Devin-M]

I recorded a short video. It's about 3:45. Started out on a bridge then descend ramp leading toward bridge. Then I turn around and climb the ramp water bottle lowered. Then I turn around, go down the ramp, then climb the ramp again, this time water bottle raised. I was able to do the climb with water bottle raised in 2 seconds less time. Uncut video.

1st climb (water bottle lowered)- 1:22 - 1:41 - 19s
2nd climb (water bottle raised)- 3:12 - 3:29 - 17s

 

[DaveC426913]

I recorded a short video. It's about 3:45. Started out on a bridge then descend ramp leading toward bridge. Then I turn around and climb the ramp water bottle lowered. Then I turn around, go down the ramp, then climb the ramp again, this time water bottle raised. I was able to do the climb with water bottle raised in 2 seconds less time. Uncut video.

1st climb (water bottle lowered)- 1:22 - 1:41 - 19s
2nd climb (water bottle raised)- 3:12 - 3:29 - 17s

There is absolutely no way to verify that you removed confounding factors - particularly whether you used the same effort on both runs and had your body positioned the same way for both runs.

If you really want to prove your point, you'll want to make a mockup - i.e. fully automated, removing yourself form the equation - where you can demonstrate that all variables are ... consistentified.

And even if you do, it still doesn't make the point - because what we're trying to tell you is that you do alter your position to compensate.

 

[A.T.]

I was able to do the climb with water bottle raised in 2 seconds less time.

See posts #2 and #104 for possible physical effects of the bottle that might improve performance.

See also many studies on how psychology influences sport performance.

 

[Drakkith]

Then I turn around, go down the ramp, then climb the ramp again, this time water bottle raised. I was able to do the climb with water bottle raised in 2 seconds less time.

So try harder on the 1st attempt and you'll do it faster. And then try even harder on the 2nd attempt and you'll do that one even faster. If you want to actually support this hypothesis you would need to get dozens of skateboarders and perform hundreds of runs where each person is giving it their all. Ideally in a 'lab' setting where you can control what skateboard they are using, the condition of the ground, remove wind and weather factors, etc.

 

[DaveC426913]

Ideally in a 'lab' setting where you can control what skateboard they are using, the condition of the ground, remove wind and weather factors, etc.

Or make a mechanical prototype and control the variables.

 

[Devin-M]

I will say on the 1st attempt without bottle extended I had to put my rear foot back on the board twice to adjust my tilt angle forward or else I would have toppled backwards. Essentially I am wondering if the skateboarder is already operating at the edge of performance without falling, does the added stability caused by changing the moment of inertia and moving center of gravity forward allow slightly more performance… like a building in an earthquake that has an active mass damper as opposed to one that doesn’t.

 

[sophiecentaur]

So try harder on the 1st attempt and you'll do it faster.

Absolutely. The human body is a really bad subject fort Physics calculations because there are just too many unknown variables - including psychology. You could possibly go faster when wearing your favourite T shirt even.

Essentially I am wondering if the skateboarder is already operating at the edge of performance without falling, does the added stability caused by changing the moment of inertia and moving center of gravity forward allow slightly more performance…

There are many unknown variables in that statement, which just makes my point. About the only thing that you can be sure of is that you can't affect the velocity or position of the Centre of Mass without applying an external force. For that you need a slope, have sticky wheel bearings or do clever clogs zig-zagging. Really hard to analyse all that.

Waving the water bottle about can change your MI and, perhaps allow you to corner differently for more thrust

 

[Devin-M]

Assuming this model is accurate, the center of mass is shown as being above where the leg joins with the hip bone. In zero G if we push the body forward from the point where the leg joins with the hip, it induces a backwards tipping torque around the COM (same is the case if friction is removed from the foot in 1 g). Back in 1 G, a bottle outstretched forward in an arm combined with the normal force on the ground induces a forward tipping torque counteracting the backward tipping torque from the push.

https://www.nature.com/articles/s41598-018-37581-9/figures/1

 

[A.T.]

a bottle outstretched forward in an arm combined with the normal force on the ground induces .

How large is the contribution of the bottle compared to what the normal force on the back foot can do, which can be almost all your weight?

 

[Devin-M]

How large is the contribution of the bottle compared to what the normal force on the back foot can do, which can be almost all your weight?

I think it depends on the weight of the bottle, and so the larger the weight of the bottle, the greater the forward tipping torque.

 

[A.T.]

I think it depends on the weight of the bottle

How much % of your bodyweight can you hold in a horizontally stretched out arm for longer than a few seconds?

 

[Drakkith]

In zero G if we push the body forward from the point where the leg joins with the hip, it induces a backwards tipping torque around the COM (same is the case if friction is removed from the foot in 1 g).

...so?

Back in 1 G, a bottle outstretched forward in an arm combined with the normal force on the ground induces a forward tipping torque counteracting the backward tipping torque from the push.

Again, so? We've already gone over this.

 

[Devin-M]

...so?

Again, so? We've already gone over this.

So the heavier the outstretched bottle, the harder you can push the body forward from the area where the leg joins the hip without her/him tipping over on a skateboard.

 

[Drakkith]

So the heavier the outstretched bottle, the harder you can push the body forward from the area where the leg joins the hip without her/him tipping over on a skateboard.

You have it backwards. A person doesn't balance on a skateboard at the center of mass, they do so on their feet. Pushing forward with the rear leg will tend to tilt the person forward. To counteract this you use your foot and toes to hold your body upright. Holding a weight forwards just makes this problem worse.

 

[Devin-M]

Pushing forward with the rear leg will tend to tilt the person forward.

That’s only if the other foot has friction (standing). On a skateboard the wheels are free to roll forward under the front foot, so pushing the body forward below center of mass causes backwards tilting torque.