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

[A.T.]

When I shred wood, I’ve found that “throwing” my arms in front of me helps me generate a greater impulse from my pushing foot on the ground, as long as I have the right timing and stay in balance.

I’d think that I’d be able to go faster on my push with added weight in my hands. Will test later.

This might be due to the mechanism I described here:

Physically, you could use the bottle to store additional momentum by accelerating it forward when the foot is pushing off, and backwards when the foot is off the ground.

 

[A.T.]

Manipulating a water bottle is a clumsy way to mimic adjusting your lean angle.

Another key difference to the Segway is the base of support. With the back leg on the ground we cannot flip over backwards, because the normal force on that leg would simply increase to counter that.

 

[A.T.]

Isn’t it then conceivable that I could get my leg further back with a water bottle in my outstretched hand, and that a longer stroke could conceivably deliver more power?

Due to the complexity of whole body biomechanics a lot of stuff is conceivable. Including that despite extending the leg slightly further, you won't get better performance.

 

[SphericalCow]

If the solution is limited not by the average output power of the kicks, but by the rider staying in balance, can the rider go faster holding the bottle and faster still if the bottle is heavier?

To accelerate:
-Lift water bottle rigidly forward with right arm
-Kick 3 times with left leg, only right foot touches board
-Lower arm and bottle

When I am making really sharp turns, I don't need a water bottle to get the minimized turn radius, I position my weight as far as I can to the side without one. The skateboard will not turn any faster, because the only way to achieve this would be for me to create a greater torque on the side I'm turning towards, but I'm already at a maximum torque such that if you handed me a water bottle I would fall over or the skateboard would slip out from under my feet.

When I am pushing, I have my weight such that I might as well just be standing with my front foot on the board -- if you handed me a water bottle and told me to stretch out my arm, I would fall over.

Now, if you were pumping your arms along with your pushes on the ground, that would be a different conversation. In terms of balance, you would not want to be holding onto a water bottle, because every balance effect you already want is done much more easily with just your body.

 

[Devin-M]

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...

 

[russ_watters]

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,
View attachment 329654

I think you are still viewing the situation backwards: his weight has to be behind his plant foot otherwise when he kicks he'll fall forwards. [Edit] Or alternately, in order for his back foot to apply a force forwards, it has to be carrying some of his weight.

and if his left knee were bent further he would seem even closer still to tipping backwards...

You wouldn't generally just bend your knee. You can see he's also bending at the ankle and hip. The net result of these motions may or may not move your COG fore/aft.

 

[jbriggs444]

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

*sigh*

We cannot tell from the picture where the center of pressure is on the shoe. The vertical blue line is just a guess. Also, it is a still picture. We cannot determine whether the posture is steady or if he is in the middle of a rotation. In particular, the state of motion of the right leg is unclear. He appears to be recovering from a kick.

If it were me, I might be extending the left leg forward a bit at the same time the right leg is kicking rearward. This would keep the net angular momentum of the body about its mass center to a minimum. In the recovery phase, the right leg would come forward and the left leg would come back, restoring a stable platform.

We know that the skater has at a number of means to regain balance even if he is currently on the verge of losing it.

1. He can extend the ankle, moving the center of pressure forward toward the toes or he can flex the ankls, moving the center of pressure rearward toward the heels.

2. He can extend the knee, moving the skateboard forward. Or he can flex the knee, moving the skateboard rearward. A similar effect can be achieved by extending or flexing at the hip or waist.

3. He can gyrate with his arms or right leg to temporarily maintain the body in an upright posture despite an unbalanced condition. This may provide time for (1) or (2) above to have an effect.

If he holds a water bottle forward, the same static balance could be achieved with a slight forward lean angle and no water bottle. If there is an advantage to be gained by holding a water bottle in the air, it is likely to do with (3) above and waving it around to help maintain stability in the torso.

 

[Devin-M]

This sums up how I was thinking of it:

 

[russ_watters]

This sums up how I was thinking of it:

View attachment 329655

Yes, I know that's what you think. Your force is pointing in the wrong direction (edit: and applied in the wrong place), so your conclusion is backwards. As I keep telling you.

A skateboard is not a hoverboard/Segway.

 

[Devin-M]

How is the force in the wrong direction?

 

[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?