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

[russ_watters]

So are you saying I can’t get a longer stroke with my leg if I move the bottle forward during the stroke compared to keeping the bottle and my hands at my sides?

1. No. Again: try it. I mean it. Right now. Stand up and lean forward, sticking your leg out horizontally behind you. You can stick your leg out horizontally without the need of a water bottle. You cannot increase your stride length by adding a water bottle/counterweight because you can already make it equal to your leg length without it.
2. That's a different claim from what you were claiming originally. Your original claim was that the leaning forward opposed the thrust, which is wrong/backwards (you have to lean backwards against your leg during the thrust, not forward away from it). If the SpaceX rocket on the transporter accelerates too fast it falls backwards, but if you accelerate too fast on a skateboard you fall forwards.

[edit] There's some additional complexity here due to body position. If the body is vertical and rigid and you apply the force below the center of gravity and the torque is larger than the opposing torque from the wheel friction you fall backwards. But when you squat and lean you align your center of gravity with the propulsion force -- and that force is far larger than the wheel friction. You're leaning back slightly to apply a forward force on the skateboard while accelerating forwards.

 

[Devin-M]

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?

 

[russ_watters]

Isn’t it then conceivable that I could get my leg further back with a water bottle in my outstretched hand,

No. Again: try it!

 

[SphericalCow]

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.

 

[Drakkith]

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?

What does the water bottle have to do with how far you can stretch you leg out? That's appears to me to mostly be a function of how close to the ground you can get your body. The further down you go, the more parallel to the ground your leg can get, which increases the distance it can push before becoming fully extended. But there is likely a point that you're too close to the ground and your leg cant bend correctly to get the optimum push, so there's probably a sweet point that's slightly different for everyone based on their exact proportions and board height.

Keep in mind that the body is perfectly capable of balancing on its own without added weight. See this image:

I just don't see any way that adding weight to your hands could help you stretch out more than you already can.

Perhaps the best evidence that adding weight to your hands doesn't help is that highly skilled professional and amateur skateboarders don't use them. Not that I know of at least.

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.

What does 'shredding wood' mean?

 

[SphericalCow]

Perhaps the best evidence that adding weight to your hands doesn't help is that highly skilled professional and amateur skateboarders don't use them. Not that I know of at least.

All actual skateboarding sports involve balance and thus require ease of arm movement. A water bottle would be terrible for ease of arm movement, so you don't see it very often.

However, OP's question is specific- they are asking about the effect a water bottle has on your ability to push on a skateboard. This is different, because the requirement for balance is much lower than when at the park or longboarding down a hill. As OP and I have noticed, holding a water bottle changes how we push on a skateboard.

What does 'shredding wood' mean?

Skateboarding.
I just tried pushing around with a water bottle. This is what happened:

I push with my right foot, and I hold the water bottle in my right hand. I face forward, center my weight, and as I start to plant my foot on the ground, I swing the water bottle ever so slightly back, so that it's to the right of my waist with my elbow in a 90 degree angle. Then, as I push off the ground, I swing my arm forward, maintaining the 90 degree angle until the bottle is in front of my sternum, and the process repeats.

It feels like I'm able to generate a greater impulse between my right foot and the ground, thus making me faster. I assume this has something to do with the increased torque from my arm, or how my weight can shift in a different way that lets me increase the friction force from my flip flop.

 

[Devin-M]

Keep in mind that the body is perfectly capable of balancing on its own without added weight. See this image:

Compared to this image the skateboarder has their knee bent as much as possible to prolong the push:

 

[Drakkith]

Compared to this image the skateboarder has their knee bent as much as possible to prolong the push:

I know. I was illustrating that a person doesn't need extra weight to balance themselves with their leg fully extended.

 

[Drakkith]

It feels like I'm able to generate a greater impulse between my right foot and the ground, thus making me faster. I assume this has something to do with the increased torque from my arm, or how my weight can shift in a different way that lets me increase the friction force from my flip flop.

I remain unconvinced, but my skepticism obviously has no bearing on whether or not this is actually true. Keep testing if you want. Grab some other skateboarders and see what you guys can come up with.

 

[A.T.]

It feels like I'm able to generate a greater impulse between my right foot and the ground, thus making me faster. I assume this has something to do with the increased torque from my arm, or how my weight can shift in a different way that lets me increase the friction force from my flip flop.

Aside from feeling not being an objective measure you have now introduced potentially insufficient traction as another parameter, beyond the balance question.

 

[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?

 

[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.