Can a skateboarder ride faster by holding a water bottle?

[Devin-M]

I was curious why I find myself naturally holding my water bottle forward while kicking on my skateboard (I would be carrying bottle in any case). That’s it. Im satisfied with the answers so far especially higher possible acceleration without toppling by increasing the mass of the counterweights on the starship booster transport system.

 

[russ_watters]

I was curious why I find myself naturally holding my water bottle forward while kicking on my skateboard (I would be carrying bottle in any case). That’s it. Im satisfied with the answers so far....

I dont think you got the right answer, because you started by providing your own wrong answer and pushed the discussion in that direction. The main reason you extending your arms is to counterbalance the weight of your extended leg when it is off the ground. It has nothing directly to do with acceleration/thrust and isnt at all related to the SpaceX crawler.

 

[Hornbein]

Again: A segway has a tilt sensor coupled to its motor electronically so that the speed is computer-controlled by the tilt. It's programmed to do that. (If it did it naturally, it wouldn't need a computer to coordinate it.)

It seems to me that there has to be a feedback mechanism built in.

 

[DaveC426913]

Imagine you are moving the starship booster and try to accelerate it too quickly from the bottom- it’s going to topple over backwards. By having a counterweight sticking out towards the front you could accelerate it faster without toppling. The weight hanging off the front causes it to want to tip over in the forward direction which counterbalances the tendency to tip over backwards if you try to accelerate it too quickly by pushing towards the bottom.

This is a bit of a red herring (i.e. while its technically true, it's not really applicable), here's why:

The booster is a rigid object. It can't deform to balance against acceleration. You can, and do.
The booster is an unstable object. It can't tilt to balance against acceleration. You can, and do.

So, what happens is: if you want to go faster, you simply crunch your body a little more or lean forward an extra couple of inches. The bottle does nothing but make that easier.

Here is the upshot:

You legs are the ultimate limiters on how fast you can go.

It's not that with the bottle you can go any faster - it's that with the bottle, you don't have to lean quite so far forward to achieve that same balance for your max acceleration. That's it.

 

[jbriggs444]

I dont think you got the right answer, because you started by providing your own wrong answer and pushed the discussion in that direction. The main reason you extending your arms is to counterbalance the weight of your extended leg when it is off the ground. It has nothing directly to do with acceleration/thrust and isnt at all related to the SpaceX crawler.

I agree. It is for a similar reason that one pumps the arms when running -- to keep the body more stable. Or why one does a preliminary lift with the hands when doing a standing broad jump -- to give the leg muscles a little more contact time and to give the body a little extra initial momentum. Or, for that matter, the same reason that one windmills the arms to save a fall from a ledge -- a temporary sink for unwanted angular momentum.

 

[russ_watters]

It's not that with the bottle you can go any faster - it's that with the bottle, you don't have to lean quite so far forward to achieve that same balance for your max acceleration. That's it.

I encourage people to try this while stationary. Pick up a fairly heavy object (water bottle, textbook, etc) and assume the position of the rider in the photo. Then extend your arm out in front of you. What happens? For me the change in balance was imperceptible and automatic.

 

[jbriggs444]

I encourage people to try this while stationary. Pick up a fairly heavy object (water bottle, textbook, etc) and assume the position of the rider in the photo. Then extend your arm out in front of you. What happens? For me the change in balance was imperceptible and automatic.

There is a similar test which could lead one to an alternate conclusion. Pick up something heavy (like a five gallon bucket of water) with the right hand and walk with it. Watch what you automatically do with the left hand.

Spoiler

You will automatically raise the left hand to reduce the leftward lean angle that your body would otherwise assume. This allows you to carry the bucket more easily without bumping it into your right hip and spilling water. [Better yet, grab a second bucket in the left hand and make half as many trips].

This behavior is not seen when holding a bucket with outstretched arm(s) in front of ones self because there is no convenient appendage that can be extended rearward. And because you may want both hands to support a bucket at arms length.

 

[Devin-M]

When I’m actually on the board with water bottle I use it in 3 situations:

-While changing turning radius
-While accelerating with foot
-While decelerating with foot

To enter right hand turn:
-Lift water bottle to the right side with right arm
-Once desired constant turn radius is achieved lower arm
-To exit right turn switch bottle to left hand, raise left arm to left until straightened out, then lower arm

To enter left hand turn:
-Lift water bottle to the left side with left arm
-Once desired constant turn radius is achieved lower arm
-To exit left turn switch bottle to right hand, raise right arm to right until straightened out, then lower arm

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

To decelerate:
-Lift water bottle rigidly behind me with left arm
-Kick 3 times to slow down with left leg, only right foot touching board
-Lower arm and bottle

 

[Devin-M]

See 5:20:

 

[russ_watters]

See 5:20:

You tell us what you see.

 

[Devin-M]

Well he says in video you “throw your upper body forward so that way you can get as much from each push as possible.”

 

[DaveC426913]

Well he says in video you “throw your upper body forward so that way you can get as much from each push as possible.”

Yes. You can do this with your body.

Assuming you are a 70kg man, you mass about 200x more than a 355mL bottle of water. So, carrying the water, you are now a 70.355kg man. Anything you can do with a 70.355kg body, you can do with a 70.000kg body.

The bottle makes it a little easier to get that 1/200th of your combined mass two feet further forward. But don't be mistaken, you would accomplish the same thing without a bottle of water by simply leaning forward an imperceptible fraction of a degree more. And by imperceptible, I mean, you won't even know you're doing it.

 

[Drakkith]

Well he says in video you “throw your upper body forward so that way you can get as much from each push as possible.”

He's not 'throwing' his upper body forward. He's leaning forward very slightly, bringing his body lower to the ground, and swinging his arms upwards and out to the front. Lowering your body lets your leg extend further backwards to push you, while bringing your arms up and forwards counteracts the down and backwards motion of your leg.

I remain convinced that your acceleration is limited by the physical ability of your legs than anything to do with balance.

 

[russ_watters]

Well he says in video you “throw your upper body forward so that way you can get as much from each push as possible.”

He's not 'throwing' his upper body forward. He's leaning forward very slightly, bringing his body lower to the ground, and swinging his arms upwards and out to the front. Lowering your body lets your leg extend further backwards to push you, while bringing your arms up and forwards counteracts the down and backwards motion of your leg.

I remain convinced that your acceleration is limited by the physical ability of your legs than anything to do with balance.

I agree with @Drakkith that there's two different things going on here and the thrust is not a function of balance. To the extent that anything is being "thrown forward" (the arms) while your foot is on the ground, it moves your center of gravity forward, increasing your speed. See also: jumping and diving. I suppose if you have a water bottle already it may help to hold it in your hands, but adding a water bottle if you didn't already have one would reduce acceleration not increase it.

After your foot leaves the ground, it's about balance - you bring your leg forward and your arms backwards to keep your COG moving at constant speed.

 

[Devin-M]

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?

 

[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?