Is it possible to get down by throwing hands upwards?

[Titania]

We are not able to move our bodies, or rather, change direction or accelerate in space without force exerting on it according to Newton's first law.

However, for example, we can throw our right hand in order to move to the left in space.

What if I throw my hands upwards in order to get down ? Is that even possible?

Do correct me if I'm wrong. And sorry for asking this silly question. Thanks in advance :)

 

[jedishrfu]

Your hands are still connected to your body, right? If yes then no you'd still be in the same spot.

Now if you took off your shoes and threw them then you'd get a reaction in the opposite direction.

 

[phinds]

If you flail your arms in space, you WILL move, but only rotationally. Your center of mass will not move but yes, you can change direction

 

[CWatters]

What if I throw my hands upwards in order to get down ? Is that even possible?

If you were floating in space and you raised your arms then yes the rest of you would move in the opposite direction. However your centre of mass won't move. When you lower your arms again you will be back in the original starting position.

 

[phinds]

If you were floating in space and you raised your arms then yes the rest of you would move in the opposite direction. However your centre of mass won't move. When you lower your arms again you will be back in the original starting position.

More to the point, if you SWING your arms, you can rotate your position so you'll be able to see more of the universe while you wait for someone to come rescue you.

 

[Titania]

I got it~ Thanks~

 

[athosanian]

I don't think you can move down just by throwing your hand up, becuase as a whole you are under no force at all. Maybe you can rotate yourself in the space, but the center of mass of your body cannot move at all.

 

[CWatters]

More to the point, if you SWING your arms, you can rotate your position so you'll be able to see more of the universe while you wait for someone to come rescue you.

See also how cats turn and land on their feet.

 

[DocZaius]

This topic reminds me of a Scientific American article from 2009 that explained how you can "swim" in curved, empty space by taking advantage of subtle effects due to general relativity:
http://www.scientificamerican.com/article/surprises-from-general-relativity/
Unfortunately, the whole article isn't available for viewing unless you're subscribed.

To be clear, this is due to GR and in the classical regime you indeed cannot move your center of mass.

 

[Dale]

I understand from many reputable authors of popular music that throwing your hands up is, in fact, the standard way to get down.

 

[and7barton]

If you are floating weightless, but within your space station... in other words, in a breathable normal atmosphere, you could propel yourself from point A to point B by cupping your hands and paddling - in other words, "Throwing" handfuls of air as you would throw water when swimming. You'd need to force the air backwards pretty hard to move yourself, but I don't see why it wouldn't work.

 

[phinds]

If you are floating weightless, but within your space station... in other words, in a breathable normal atmosphere, you could propel yourself from point A to point B by cupping your hands and paddling - in other words, "Throwing" handfuls of air as you would throw water when swimming. You'd need to force the air backwards pretty hard to move yourself, but I don't see why it wouldn't work.

Yes, it would work, but that's not what this thread is about.

 

[Vanadium 50]

subtle effects due to general relativity:

And we're back to - almost literally - using GR to solve an inclined plane problem.

 

[jedishrfu]

With the preceding segway, thank you @Vanadium 50:

The YMCA is offering a course for future GR space swimmers who are tired of incline plane problems:

http://iopscience.iop.org/1367-2630/8/5/068/media/movie1.avi

based on a paper by Dr Jack Wisdom

http://dspace.mit.edu/bitstream/handle/1721.1/6706/AIM-2002-017.pdf?sequence=2

 

[Alec Dacyczyn]

*Cue mid-90s hip-hop references*

Sorry. I had too.

 

[sophiecentaur]

You'd need to force the air backwards pretty hard to move yourself, but I don't see why it wouldn't work.

Probably best to blow and use your mouth / lungs as a rocket engine. The power is better matched to the situation.

 

[stevmg]

Actually, you could move by expending work. Not enough space or time here to go into the classical Newtonian physics regarding this. The body does have energy (work) available in the form of ATP, etc. which muscles can use to generate motion and can be directed. With that, motion can be created.

 

[phinds]

Actually, you could move by expending work. Not enough space or time here to go into the classical Newtonian physics regarding this. The body does have energy (work) available in the form of ATP, etc. which muscles can use to generate motion and can be directed. With that, motion can be created.

Not when there's nothing to work ON. You would have to eject mass to change your center of gravity and absent that, it won't move.

 

[jackwhirl]

More to the point, if you SWING your arms, you can rotate your position so you'll be able to see more of the universe while you wait for someone to come rescue you.

As demonstrated in SmarterEveryDay 85.

See also how cats turn and land on their feet.

That too!

 

[stevmg]

As demonstrated in SmarterEveryDay 85.

That too!

Remember, the body (cats, too) have "potential energy" in the form of ATP and actin which can be expressed as work and create motion. The actual motion is dependent on the living tissue. F = ma. s = (1/2 ) at^2. Depending on t [the time of application of the force] one can back calculate the work expended [from the ATP] to make this motion. Living tissue can generate motion [motion from a point A to point B] because there is potential energy stored up which can be converted to kinetic energy and motion as required.

 

[phinds]

Remember, the body (cats, too) have "potential energy" in the form of ATP and actin which can be expressed as work and create motion. The actual motion is dependent on the living tissue. F = ma. s = (1/2 ) at^2. Depending on t [the time of application of the force] one can back calculate the work expended [from the ATP] to make this motion. Living tissue can generate motion [motion from a point A to point B] because there is potential energy stored up which can be converted to kinetic energy and motion as required.

And how does that happen when there is nothing to push against? You did not answer my previous post. Do you dispute my statement that no amount of work by a free-falling body in space can change the position of its center of mass, absent ejecting some amount of mass?

You keep saying "do work". Do work against WHAT?

 

[jackwhirl]

Remember, the body (cats, too) have "potential energy" in the form of ATP and actin which can be expressed as work and create motion.

But... ATP is a molecule that stores chemical energy, not potential energy. You're right that our bodies can use it to produce motion through our muscles, but as phinds has pointed out, twice now, that motion can't actually move the center of mass without something to push against, in accordance with Newton's first and third laws.
Edit: third, not second, sorry.

 

[Doc Al]

Living tissue can generate motion [motion from a point A to point B] because there is potential energy stored up which can be converted to kinetic energy and motion as required.

Only if an external force is exerted on the "living tissue".

If you think otherwise, please provide a specific example.

 

[itfitmewelltoo]

If you did try blowing air, you are going to have to rotate 180 to breath in or you are defeating the purpose.

Urinating would expel more mass that blowing air.

 

[phinds]

If you did try blowing air, you are going to have to rotate 180 to breath in or you are defeating the purpose.

Urinating would expel more mass that blowing air.

Yeah, good point, but in space, if you open your fly bad things might happen to your privates

 

[mfb]

If you did try blowing air, you are going to have to rotate 180 to breath in or you are defeating the purpose.

You don't have to, breathing in takes air from all directions and generates nearly no thrust.

 

[stevmg]

Only if an external force is exerted on the "living tissue".

If you think otherwise, please provide a specific example.

The external force comes from the conversion of potential energy to kinetic energy. Imagine taking a deep breath. Then being placed in space. Then forcibly exhaling and this action-reaction would push the individual the opposite direction of the exhalation.
Similar to a CO2 cartridge in space, at first "immobile," then the creation of a hole in the tip. The action-reaction to the CO2 rushing out would be motion of the cartridge in the opposite direction when before there was no motion of the cartridge. Note than in this case, too, there is NO external force applied to the cartridge other than what subsequently comes from the potential energy that was contained in the cartridge itself.

 

[Doc Al]

The external force comes from the conversion of potential energy to kinetic energy. Imagine taking a deep breath. Then being placed in space. Then forcibly exhaling and this action-reaction would push the individual the opposite direction of the exhalation.

The external force comes from simply expelling the air! As long as you are willing to "expel" something, you can create an external force. Try doing it without expelling anything. You still have plenty of potential energy stored inside you. Where's the external force now? (Of course, you can always break a piece off and toss it, but that is trivial.)

 

[phinds]

The external force comes from simply expelling the air! As long as you are willing to "expel" something, you can create an external force. Try doing it without expelling anything. You still have plenty of potential energy stored inside you. Where's the external force now? (Of course, you can always break a piece off and toss it, but that is trivial.)

This point has been made repeatedly to stevmg, but he ignores it and even failed to answer my direct question about it in post #21

 

[Doc Al]

This point has been made repeatedly to stevmg, but he ignores it and even failed to answer my direct question about it in post #21

Indeed. I eagerly await his response to your direct question. So far, I only see tap dancing around it.

 

[A.T.]

Similar to a CO2 cartridge in space, at first "immobile," then the creation of a hole in the tip. The action-reaction to the CO2 rushing out would be motion of the cartridge in the opposite direction when before there was no motion of the cartridge. Note than in this case, too, there is NO external force applied to the cartridge other than what subsequently comes from the potential energy that was contained in the cartridge itself.

If the force from the expelled gas on the cartridge isn't considered to be external, then cartridge and expelled gas are considered one object, whose center of mass doesn't move.

 

[phinds]

If the force from the expelled gas on the cartridge isn't considered to be external, then cartridge and expelled gas are considered one object, whose center of mass doesn't move.

Excellent point.

 

[Titania]

I don't think you can move down just by throwing your hand up, becuase as a whole you are under no force at all. Maybe you can rotate yourself in the space, but the center of mass of your body cannot move at all.

Isn't throwing hands upwards considered as an action? ...I'm confused.

 

[Doc Al]

Isn't throwing hands upwards considered as an action?

What do you mean?

 

[Titania]

I'm sorry I sound a bit silly.. But u see, to move your body in space, u will have to exert force or do an action, right? Isn't throwing arms upwards considered as an action? Do correct me.

 

[jackwhirl]

I'm sorry I sound a bit silly.. But u see, to move your body in space, u will have to exert force or do an action, right? Isn't throwing arms upwards considered as an action? Do correct me.

Of course that is an action. The issue you'll run into, if you want to actually travel some distance in space, is that your arms are typically attached to you. If you throw your arms up, that is an action causing an equal and opposite reaction in the rest of your body. But, then your arms reach maximum extension and the force of tension in your arms brings them to a stop, along with the rest of your body which was, briefly, traveling in the opposite direction. Stretching out could let you reach something you couldn't before, but it won't let you go anywhere.

 

[Titania]

Of course that is an action. The issue you'll run into, if you want to actually travel some distance in space, is that your arms are typically attached to you. If you throw your arms up, that is an action causing an equal and opposite reaction in the rest of your body. But, then your arms reach maximum extension and the force of tension in your arms brings them to a stop, along with the rest of your body which was, briefly, traveling in the opposite direction. Stretching out could let you reach something you couldn't before, but it won't let you go anywhere.

Okay~ Thanks :)

 

[phinds]

I'm sorry I sound a bit silly.. But u see, to move your body in space, u will have to exert force or do an action, right? Isn't throwing arms upwards considered as an action? Do correct me.

Yes, this is correct (I'm assuming you mean "rotate", not move your center of mass). See post #5. This is what everyone in the thread, except stevmg, have said throughout.