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

[Devin-M]

Sorry for the ridiculous title but this is actually a serious question.

Suppose we have a skateboard rider, right foot forward, left foot kicking. In one case the rider extends their arm directly forward holding a water bottle, in the other case they don’t. 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?

If the answer is yes, I suspect the bottle counterbalances the torque on the rider’s body induced by the ground kicks, so the heavier bottle enables more powerful ground kicks without the rider toppling.

 

[A.T.]

If the solution is limited not by the average output power of the kicks, but by the rider staying in balance,

This is way too vauge. How would the bottle achieve anything in terms of balance that cannot be achieved by changing the body pose? Biomechanic efficiency is very complex.

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.

 

[hmmm27]

Could use it to get deeper digs when slaloming... but then there's the increased overall mass.

 

[DaveC426913]

I don't see how adding an extra mass could allow the rider could go faster, but I can see how it might allow them to balance and corner better. The water bottle is functionally comparable to a counterbalancing big tail on a squirrel or cheetah.

(Although I question how effective a 355g mass is to a 70,000g person.)

 

[Devin-M]

Doesn’t the rider need some weight forward to prevent the kicks toppling them backward? For example a one wheel electric board, the more the rider leans forwards the faster the constant speed. For example gravity trying to topple the rider forward while the force from the wheel prevents this.

 

[DaveC426913]

Doesn’t the rider need some weight forward to prevent the kicks toppling them backward? For example a one wheel electric board, the more the rider leans forwards the faster the constant speed. For example gravity trying to topple the rider forward while the force from the wheel prevents this.

Well, his whole body is going to balance that out. It's about 200x more massive than the bottle.

 

[Devin-M]

On a one wheel electric board, if the only change was putting the bottle forward would it be faster?

 

[DaveC426913]

On a one wheel electric board, if the only change was putting the bottle forward would it be faster?

No amount of added mass will make any rider/vehicle faster**.

However, as A.T. points out, biomechanical efficiency is very complex, and it is certainly plausible that a rider might use a small mass on the end of his arm to help him balance a little better - again - like a cheetah uses their massive tail as a counter-balance when cornering.

I'd say performance on such vehicles is certainly a matter of practice, skill and a lot of personal style. Some certain number of riders might feel a counter-weight helps them perform a fraction better, but I think that's on a case-by-case basis - by that I mean I doubt one could prove that, objectively, a rider carrying a bottle could outperform a rider without a bottle - all other things being equal.

But I am neither a biophysicist, nor a board-rider, so I speak from ignorance.** excepting, perhaps, parachutes and the like

 

[Devin-M]

Consider an inverted pendulum fixed to a segway or one wheel board that has a robotic arm able to lift a water bottle forward. With the bottle lowered the pendulum is balanced and segway/one wheel board is stationary but with the bottle raised forward the pendulum is out of balance, leans forward and the segway / one wheel board begins to move in the direction the water bottle was lifted.

 

[Devin-M]

 

[jbriggs444]

Consider an inverted pendulum fixed to a segway or one wheel board that has a robotic arm able to lift a water bottle forward. With the bottle lowered the pendulum is balanced and segway/one wheel board is stationary but with the bottle raised forward the pendulum is out of balance, leans forward and the segway / one wheel board begins to move in the direction the water bottle was lifted.

The idea is silly.

A Segway or one wheel board is going to be limited by motor torque and wind resistance. And, of course, the speed limiter. Once you've disabled that and paid up your life insurance, you would want to attain the maximum stable forward lean angle in a crouch to reduce wind resistance and tilt the mechanism to maximize forward thrust.

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

 

[Devin-M]

Would it take more torque from the wheel to counteract the torque of the leaning pendulum if the pendulum began telescoping so it was longer (weight concentrated at the end of pendulum)? Suppose the pendulum is already leaning as far forward as possible without touching the ground (or 90 degrees from vertical).

 

[jbriggs444]

Would it take more torque from the wheel to counteract the torque of the leaning pendulum if the pendulum began telescoping so it was longer (weight concentrated at the end of pendulum)? Suppose the pendulum is already leaning as far forward as possible without touching the ground (or 90 degrees from vertical).

The height of the pendulum is irrelevant to the torque balance. Only the angle to the mass center matters.

 

[Devin-M]

Wouldn’t a longer pendulum act like a longer lever on the fulcrum at the hub, generating more torque?

 

[Drakkith]

I don't understand the significance of the pendulum and segway example. What does this have to do with a person riding a skateboard?

 

[DaveC426913]

with the bottle raised forward the pendulum is out of balance, leans forward and the segway / one wheel board begins to move in the direction the water bottle was lifted.

A segway has a tilt sensor coupled to its motor electronically so that the speed is computer-controlled by the tilt. That has nothing to do with a human-powered skateboard.

Wouldn’t a longer pendulum act like a longer lever on the fulcrum at the hub, generating more torque?

Are you swinging the water bottle in a circle?
Do you accept that these skaters are both balanced and neither will go faster than the other?

 

[Devin-M]

Do you accept that these skaters are both balanced and neither will go faster than the other?

I suspect the 2nd rider may have kicked harder to retain balance as the longer arm acts like a telescoping inverted pendulum.

 

[Drakkith]

Suppose we have a skateboard rider, right foot forward, left foot kicking. In one case the rider extends their arm directly forward holding a water bottle, in the other case they don’t. 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?

People don't hold their arms out front when kicking in the first place, so we're already starting off with a situation that people aren't really used to. Instead, they lean forward and bend their knee slightly, holding their arms down and out to the side, which keeps their center of mass both centered over the board and lowers it closer to the ground, making it more difficult for the person to topple over. See below image.

While it is certainly possible to hold a water bottle (or another object) in your hand while skateboarding, my own experience with various physical activities similar to this leads me to believe that it would actually make balancing and using the skateboard more difficult. Especially since a single water bottle introduces an asymmetry and probably makes it more difficult to balance left/right. Increasing someone's weight and making the load asymmetrical tends to make things harder, not easier.

 

[DaveC426913]

While it is certainly possible to hold a water bottle (or another object) in your hand while skateboarding, my own experience with various physical activities similar to this leads me to believe that it would actually make balancing and using the skateboard more difficult. Especially since a single water bottle introduces an asymmetry and probably makes it more difficult to balance left/right. Increasing someone's weight and making the load asymmetrical tends to make things harder, not easier.

I'd think the asymmetry would be too small to make a difference, its not like we have trouble walking carrying a bottle.

On the other hand, regardless of which hand its in, having a weight on the end of an appendage would allow for some balance leverage, a little like a tightrope walker with a pole.

 

[DaveC426913]

I suspect the 2nd rider may have kicked harder to retain balance as the longer arm acts like a telescoping inverted pendulum.

Sure, but the point is, it doesn't contribute to going faster.

 

[Devin-M]

Sure, but the point is, it doesn't contribute to going faster.

Are you saying if they kicked harder it didn’t make them go faster?

 

[DaveC426913]

Are you saying if they kicked harder it didn’t make them go faster?

Of course it would - either of them would go faster if they kicked harder.
The water bottle and extended arm is irrelevant.

 

[Devin-M]

Im curious why it seems to work for inverted pendulum on segway but not skateboard? Once the inverted pendulum is horizontal, and the wheel is outputting enough torque to keep it at that angle, the only way to go faster at constant speed seems to telescope the pendulum as that requires greater counter-torque from wheel to maintain angle.

 

[russ_watters]

Looks to me like skateboarders have to shift their cog backwards to counteract the counter-clockwise torque of the foot-pedaling force (which is mostly up, not forward).

But this thread is really weird. Such a bizarre and seemingly pointless question.

 

[Hornbein]

Im curious why it seems to work for inverted pendulum on segway but not skateboard? Once the inverted pendulum is horizontal, and the wheel is outputting enough torque to keep it at that angle, the only way to go faster at constant speed seems to telescope the pendulum as that requires greater counter-torque from wheel to maintain angle.

View attachment 329616

"go faster at constant speed" You mean go faster at constant angle?

 

[Devin-M]

"go faster at constant speed" You mean go faster at constant angle?

Yes won’t telescoping the inverted pendulum allow faster speed at constant angle?

 

[Hornbein]

Yes won’t telescoping the inverted pendulum allow faster speed at constant angle?

You mean increasing the length (telescoping could go either way). Seems like it would to me.

My skateboarding experience is too limited to say much in this thread. But it seems to me the idea is that the waterbottle increases the angular inertia of the rider, thus allowing stronger strokes.

 

[DaveC426913]

Im curious why it seems to work for inverted pendulum on segway but not skateboard?

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.)

 

[Devin-M]

Yes, but the farther the inverted pendulum length extends (assume pendulum constant angle parallel to ground), the motor in the wheel needs to generate more torque to maintain the constant angle, leading to acceleration until the wind drag balances with increased torque at greater constant speed as the pendulum length increases.

 

[DaveC426913]

Returning to the opening question:

... can the rider go faster holding the bottle and faster still if the bottle is heavier?

No.

 

[Drakkith]

I'd think the asymmetry would be too small to make a difference, its not like we have trouble walking carrying a bottle.

Hey I didn't say the effect was large...

Yes, but the farther the inverted pendulum length extends (assume pendulum constant angle parallel to ground), the motor in the wheel needs to generate more torque to maintain the constant angle, leading to acceleration until the wind drag balances with increased torque at greater constant speed as the pendulum length increases.

I still don't understand what the segway example has to do with this.

Doesn’t the rider need some weight forward to prevent the kicks toppling them backward? For example a one wheel electric board, the more the rider leans forwards the faster the constant speed. For example gravity trying to topple the rider forward while the force from the wheel prevents this.

Don't these use gyroscopes to balance the board and rider?

 

[Devin-M]

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.

 

[jbriggs444]

Imagine you are moving the starship booster and try to accelerate it too quickly from the bottom- it’s going to topple over backwards.

This is a far cry from the man on a skateboard or the man on a Segway that you've been speaking of. In both of those cases, there is a very active suspension in place.

If performance over a timed quarter mile is your goal for safe booster transport then one could equally well put hydraulic pads under the thing and tilt it forward at a crazy angle to get the same effect as a guy standing on top with a water bottle in his out-stretched hand.

If I were the engineer in charge, I would not be very receptive to ideas which involve attaching gear to the booster or in filling the craft with shiftable fluids. I would have to think long and hard before engineering a dynamic support system instead of a rigid attachment to the conveyor. Fewer failure modes to anticipate. However, tire pressure would be something important to look at. Those look like more or less ordinary pneumatic tires. Non-rigid. If you've ever built a tower from foam blocks, you may understand the stability problems that could result.

 

[Devin-M]

Using current booster transport setup, could we achieve greater acceleration without toppling by increasing the mass of the counterweights?

 

[russ_watters]

Using current booster transport setup, could we achieve greater acceleration without toppling by increasing the mass of the counterweights?

Sure. Not sure why they would want to, but sure.

Where is this thread going? Was this the point the whole time? The meandering/lack of focus is really off-putting and doesn't lend itself to quality discussion. If this rocket wasn't the point, please get to the point rapidly.

 

[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?