How Can Human Bodies Do Reaction Control Movement In Space Without RCS?

[kerbiloid]

2 hours ago, AckSed said:

we're assuming that this works by shifting their centre of mass

The CoM doesn't play a role. The rotational momentum conservation law does.

If you hang a mini electromotor in zero-G, and engage it, the engine body will start counter-rotating opposite to its rotor, while the CoM stays unchanged.

https://en.wikipedia.org/wiki/Reaction_wheel

That's why you must hold an electric drill strongly, otherwise it will counter-rotate into you with its handle.

[AckSed]

Thinking out loud here, and trying to say why this is still troubling me: we - humans - do not have an internal component that can spin freely about an axis and that tries to apply a counter-force to the outside frame. We have levers attached to hinges. Sure, the momentum transfer of bringing one arm in can impart a force that rotates. But can we impart a net positive rotation or will swinging our arm out again exert an equal force that exactly matches and brings us back to the start?

In both cases, either the drill of your example, a set of gyroscopes or our muscles do use external energy (electricity or stored chemicals), so it may not match to my classical Newtonian intuition. Which is why I want to see it tried.

[Kerwood Floyd]

<Retracted>

 

1 hour ago, AckSed said:

In this context rotation is moving along an axis with a start point and desired end point, while spin is uncontrolled, continuous rotation.

Along an axis or about an axis?

Edited November 27 by Kerwood Floyd
Too quick

[AckSed]

23 minutes ago, Kerwood Floyd said:

<Retracted>

 

Along an axis or about an axis?

About an axis, my bad.

[Lisias]

14 hours ago, Pthigrivi said:

Exactly. Thats the premise of a reaction wheel. A person sitting in an office chair can rotate themselves about by rotating their waist slow then fast but I think this is only because they’re abusing the coefficients of static and kinetic friction where the shaft meets the sleeve at the base of the chair. In space everything would cancel out. You need to rotate a mass fully around your center of gravity to rotate. Im confident passing a dumbbell around your waist is mechanically similar to an electric reaction wheel. The total rotation on the system remains static but the mass fraction that is your body can be manipulated by rotating the dumbbell opposite. Im less certain about doing somersaults by windmilling your arms. Do the forces on your shoulder joint cancel out? Or does the rotation of the mass fraction of your arms create an opposite rotation in the rest of your body?

What about Moment of Inertia? With weights in your hands and spreading the arms wide open, the Moment of Inertia of your body above the waist would be way bigger than below it. So, by rotating your wait, your legs will turn and not your chest.

Then you invert: close your arms to your body and do a Van Damme - again with weights in the ankles. Now the lower portion of your body will have a way bigger Moment of Inertia than the upper body. So the upper body will rotate.

 

4 hours ago, AckSed said:

The question we're asking is: is this true?

Equation of moment of Inertia:

Quote

In general, given an object of mass m, an effective radius k can be defined, dependent on a particular axis of rotation, with such a value that its moment of inertia around the axis is I = m k 2 , where k is known as the radius of gyration around the axis.

https://en.wikipedia.org/wiki/Moment_of_inertia

So the I scales quadratically in relation to the radius. So a dumbbell weighting 10Kg will have a Moment Of Inertia of

I = 10Kg * 10²cm (more or less)

At your hands when you close your arms into your chest, and

I = 10Kg * 88²cm (in my case, I just measured it)

When you wide open your arms. Or:

I_closedarms = 0.1 kg⋅m²

I_openarms = 7,744 kg⋅m²

So it's obvious that my chest will offer way less resistance due inertia with my arms wide open.

If I connect two different Moment of Inertia on a gear on a friction less environment (It doesn't need even to be gravity less), and that gear applies torque on one of them, the one that will rotate will be the one with less inertia, right?

I think we can easily prove this theory by using one of that Gym "rotary torso machines":

Since your upper body is way heavier than your lower, if you twist your hips without securing your hands on the handles (not exactly advisable, they are there for a reason), your lower body will rotate because its inertia (and even the friction on the gears you are over) will be less than the upper body Moment of Inertia.

Now tie some weights in your legs to make your lower body be heavier than your upper body and repeat the experiment.

And there's also Tightrope Walking:

The stick is there exactly to add Moment of Inertia to the walker, making easier to correct the attitude.

All of them are the same concept: using Moment of Inertia to induce torque to counteract Inertia and even Gravity.

I don't see why this would not work in space.

 

14 hours ago, Pthigrivi said:

Im confident passing a dumbbell around your waist is mechanically similar to an electric reaction wheel. The total rotation on the system remains static but the mass fraction that is your body can be manipulated by rotating the dumbbell opposite. Im less certain about doing somersaults by windmilling your arms. Do the forces on your shoulder joint cancel out? Or does the rotation of the mass fraction of your arms create an opposite rotation in the rest of your body?

That would be a smarter way to accomplish what I had proposed, I think.

But the way I suggested allow a direct mapping to the Moment of Inertia equations and would be easier to validate or falsify mathematically.

[Shpaget]

White total angular momentum can't be changed, it is entirely possible to arbitrarily change orientation. Here's a video demonstration. No rotation at the beginning, spin up then spin down and finally no rotation in a completely opposite orientation.

 

 

Edited November 27 by Shpaget

[Superfluous J]

11 hours ago, Kerwood Floyd said:

Can you explain to me the difference between rotation and spin?

Turn your head left. You rotated it.

Kick yourself to the left in your office chair. Assuming your knees don't bang into your desk, you're now spinning.

[Lisias]

6 hours ago, Shpaget said:

White total angular momentum can't be changed, it is entirely possible to arbitrarily change orientation. Here's a video demonstration. No rotation at the beginning, spin up then spin down and finally no rotation in a completely opposite orientation.

Oukey. So people here spent the whole day making calculations, doing philosophy and proposing experiments to be done by astronauts on ISS.

And you prove the point by dropping a cat? Don't you have a heart?

[kerbiloid]

Cats are laughing at this thread.

They use tail as the attitude control in free fall.

Spoiler

 

[Shpaget]

Tail probably helps, but it's not necessary, according to more cats being thrown, this time in the vomit comet.

 

[Snark]

Going back to the title of the thread:

Quote

How Can Human Bodies Do Reaction Control Movement In Space Without RCS?

The answer is simple:

They do it the exact same way that spacecraft with reaction wheels do.  You just use your built-in biological reaction wheels.  They're called "arms and legs".

If you're floating motionless in space, for example, and you windmill your arms in one direction, your body will rotate in the opposite direction because your net angular momentum has to remain at zero.  You're leveraging the principle of conservation of angular momentum.

You also have the ability to increase or decrease the moment of inertia of your arms and legs based on how far out from your CoM you hold them, and you can leverage that, too,  For example:

1. Stick your arms out in front of you, straight, and sweep them to the left:  your body rotates to the right.  (By a smaller amount, because your body's a lot more massive than your arms.)
2. Then you pull your arms in close to your body and sweep them back over to the right again.  It'll make your body rotate left, yes... but your arms have a lower moment of inertia when you're sweeping them right, so your body won't rotate as much to the left as it rotated right during step 1, above.
3. Voila, your limbs are back to their starting position, and you're still not rotating, but your body has rotated slightly to the right from when you began.  (First it rotated right, then it rotated left by a smaller amount).
4. Repeat the above steps to rotate around as much as you like.

On 11/27/2024 at 9:56 AM, AckSed said:

Thinking out loud here, and trying to say why this is still troubling me: we - humans - do not have an internal component that can spin freely about an axis and that tries to apply a counter-force to the outside frame. We have levers attached to hinges. Sure, the momentum transfer of bringing one arm in can impart a force that rotates. But can we impart a net positive rotation or will swinging our arm out again exert an equal force that exactly matches and brings us back to the start?

We can't spin our joints like wheels, no.   But it works.  To illustrate, I've colored your statement above:  green for the correct statements, and red for the incorrect.

Swinging the arm back to its start position does exert a force back in the the opposite direction, and does bring you back towards the start.  But you can jigger it so it doesn't bring you back as much to the start as you originally moved, and so you end up with a net change in orientation when you complete the operation.

You can do this by "putting your thumb on the scales", which you do by changing your limb's moment of inertia.  Which you can do by bringing it closer or farther to your CoM.  You arrange things so that your limb has a big moment of inertia when you move it one way, and a smaller moment of inertia when you move it the opposite.

Your net angular momentum remains zero throughout the process-- when your arms are moving one way, the rest of you is moving the opposite.  So you can't use this technique to change your net angular momentum.  For example, if you were sitting at rest to begin with, you can't end up spinning round and round.  Imparting net angular momentum like that would require expending reaction mass, such as RCS.  It's the exact same limitation that reaction wheels on spacecraft have, and is why spacecraft need to have RCS even if they also have reaction wheels.

• All of this is the reason why cats always land on their feet when dropped.
• It's why gymnasts and acrobats and people diving off a high diving board can control precisely what orientation they land in, after doing multiple flips and such in midair.  (Yes, they carefully control the spin when they launch... but they also actively control while in flight).
• You do a lot of it when you walk-- it helps you maintain balance.  It's just that your balance is good enough (since your brain's designed for it) that you don't notice the little automatic corrections your body makes to keep you from toppling over.

Ever watch someone learning to ride a unicycle?  In the early days, before they become really proficient and their balancing is still pretty tenuous, you'll notice them twisting their body and windmilling their arms around a lot in order to maintain balance. That's what they're doing there, too-- using their arms as reaction wheels to help with balancing.

(Being a unicyclist myself, I can attest that it works.) 

On 11/27/2024 at 9:56 AM, AckSed said:

In both cases, either the drill of your example, a set of gyroscopes or our muscles do use external energy (electricity or stored chemicals), so it may not match to my classical Newtonian intuition. Which is why I want to see it tried.

That's actually irrelevant to this discussion.  Be very careful not to confuse energy with momentum (either angular or linear).  They're completely different things.  Nothing in this discussion has anything to do with where energy comes from-- this whole discussion is about momentum (specifically, angular momentum)-- so don't get distracted by red herrings such as energy.

 

On 11/26/2024 at 8:17 PM, Superfluous J said:

It's not a misunderstanding of physics. It's a misunderstanding between two people communicating.

No, you cannot add angular momentum to yourself if you're floating in space.

Yes, you can (if you're basically sitting still) change your orientation to any way you want. It's not easy (especially in a bulky space suit) but it is absolutely possible.

Quoted for brevity and truthiness.  This, exactly this.  What he said. 

(Other people on the "yes, you can" side of this argument are correct, too.  I'm just quoting this one because it's nice and short, a quality that I notoriously lack.)

 

[Vanamonde]

Fie. 

[kerbiloid]

On 11/28/2024 at 6:50 PM, Shpaget said:

Tail probably helps, but it's not necessary, according to more cats being thrown, this time in the vomit comet.

They rotate the rear end of the torso, and the tail is its distant part, with the greatest rotation amplitude and frequency.

***

Also:
https://i.sstatic.net/oBHlfm.png