Can a skateboarder ride faster by holding a water bottle?

[Devin-M]

Assume it’s an electric skateboard accelerating from left to right. With no bottle and enough acceleration, the top of the booster topples towards the left.

 

[jbriggs444]

How is the force in the wrong direction?

The force of ground on foot is upward and forward, not purely forward. It is hard to speculate precisely about its line of action -- which will move around during the kick anyway.

 

[russ_watters]

Assume it’s an electric skateboard accelerating from left to right. With no bottle and enough acceleration, the top of the booster topples towards the left.View attachment 329658

This is better, but it is different from a skateboard propelled by the rider. You're still ignoring the force vector due to the weight of the rider though. The wheels apply a force up against that.

 

[Devin-M]

What I’m suggesting is the bottle held out front would allow someone riding an electric skateboard to accelerate faster (without toppling).

 

[russ_watters]

What I’m suggesting is the bottle held out front would allow someone riding an electric skateboard to accelerate faster (without toppling).

Then why have we been talking about a skateboarder propelling himself this whole time?

....but in either case, no it wouldn't.

 

[Devin-M]

Because the force propelling the rider on a non electric board is similarly being applied below her/his center of mass, leading to a torque on the body tending to topple them backwards.

 

[A.T.]

Assume it’s an electric skateboard accelerating from left to right.

An electric skateboard accelerates the rider via a force, which exerts a backward tipping torque on the rider.

On a normal skateboard the rider accelerates the skateboard, so no such backward tipping torque exists from acceleration.

The forces between rider and board have opposite directions in the two situations.

 

[Devin-M]

Even on a normal skateboard the force propelling the rider is applied below the center of mass, still leading to a backwards toppling torque on the body.

 

[russ_watters]

Even on a normal skateboard the force propelling the rider is applied below the center of mass, still leading to a backwards toppling torque on the body.

No!

Please do this:
Stand up. Take one step forward and stop in the middle with your body vertical and exactly between your front and back foot. Each foot is carrying half your weight and also applying a force towards your body. @jbriggs444 is right that biomechanics make this complicated, but the lowest energy/effort scenario is where you are not applying any forces with your muscles. In that case your legs and the floor form a truss and the forces are transmitted linearly through your legs. [If you try this on slippery ground your legs will slip but you can apply a force to rotate your legs closed to prevent that, changing the angle of the force. Or you may find yourself in an unintentional split.]

Now shift your weight forward until it is centered over your front foot. Lift your back foot and tap it on the ground. You will find your body angle is forward to counterbalance your back leg, but the COG is centered over your front foot. Notice you are unable to apply a significant force with your back foot. In order to do so you have to rotate your body/shift your COG backwards.

@jbriggs444 is also right that dynamic motions make this more complicated and can vary but in general the simplest motion of the COG of the skateboard rider is one where he is falling backwards (COG behind his plant foot), pushes himself back almost upright with his power stroke and then falls backwards again.

 

[A.T.]

Because the force propelling the rider on a non electric board is similarly being applied below her/his center of mass, leading to a torque on the body tending to topple them backwards.

You are forgetting the vertical force component again, which acting on the back foot that is pushing off can counter the torque from the horizontal component.

In other words: The ground reaction force vector can still pass behind the center of mass, creating forward tilting torque (balanced by the front foot on the board).

 

[SphericalCow]

If we draw a vertical line through his ankle and look at how much mass is on each side of this line, to my eye he looks very close to tipping over backwards, and if his left knee were bent further he would seem even closer still to tipping backwards...
View attachment 329654

When you are recovering from a push, the skateboard's acceleration catches up to the COM's acceleration, and there is no net torque on the body. He is standing on one pivot point. Handing him a water bottle would do nothing he isn't already doing with his arms, that's why they're so close to his body.

If you were to analyze the net torque of his push on his body, you'd find that the net torque is zero/ tiny. Good skaters are able to make the complex body movements such that the torque produced from a push doesn't affect their balance, and they do it without a water bottle.

The water bottle does nothing for your balance that you can't already do more easily with your arms.
You should loosen your trucks.

 

[Devin-M]

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.

 

[DaveC426913]

Because the force propelling the rider on a non electric board is similarly being applied below her/his center of mass, leading to a torque on the body tending to topple them backwards.

So you lean forward an imperceptible amount. That balances out the acceleration every bit as effectively as a tiny extra weight in front.

These two orientations are equally balanced. The left one just happens to mass 1/200th more*.

*which, by the way, will reduce your acceleration

 

[Devin-M]

Standing on the floor, if I tried to stretch my left leg (toe touching ground) as far back as possible while still being able to lift the toe of said leg off the ground, could I lift it while it’s stretched further back if I had a water bottle in my right forwardly outstretched arm?

 

[jbriggs444]

Standing on the floor, if I tried to stretch my left leg as far back as possible while still being able to lift to of said leg off the ground, could I lift it while it’s stretched further back if I had a water bottle in my right forwardly outstretched arm?

That would be a hideously inefficient posture from which to propel a skateboard. You do not want to maximize the extension of the left leg behind your center of gravity. You want to maximize the horizontal component of thrust (times frequency). Extreme postures work against this goal.

That said, yes, if you hold a water bottle in your outstretched hand, that shifts your center of gravity slightly toward the hand and away from the feet, giving you the ability to extend the left leg back further behind the balance point.

In any case, it seems that you are insistent proving a point rather than having a worthwhile discussion. So I am out.

 

[Drakkith]

Standing on the floor, if I tried to stretch my left leg (toe touching ground) as far back as possible while still being able to lift the toe of said leg off the ground, could I lift it while it’s stretched further back if I had a water bottle in my right forwardly outstretched arm?

The water bottle does nothing that can't be done by simply leaning forward slightly.

 

[Devin-M]

I was able to lift my rear foot's toe off the ground about 10 inches further back while holding a water bottle with the same front foot placement on flat ground...

 

[russ_watters]

I was able to lift my rear foot's toe off the ground about 10 inches further back while holding a water bottle with the same front foot placement on flat ground...

It isn't clear to me what you think that means.

 

[Devin-M]

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

 

[DaveC426913]

I was able to lift my rear foot's toe off the ground about 10 inches further back while holding a water bottle with the same front foot placement on flat ground...

This is not a valid test, for a couple of reasons.

1. Where are you measuring from? See, it doesn't matter if your leg can extend farther from some fixed reference point with or without the bottle. You can just drop the water bottle and lean a little bit further forward. Your absolute measurement to some fixed point is not relevant. You'd want your absolute measurement from your centre of mass - which moves.
2. Perhaps a bit of confirmation bias happening here?

 

[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.