# Is it possible to get down by throwing hands upwards?

1. Aug 11, 2015

### 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 :)

2. Aug 11, 2015

### Staff: Mentor

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.

3. Aug 11, 2015

### 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

4. Aug 12, 2015

### CWatters

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.

5. Aug 12, 2015

### phinds

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.

6. Aug 12, 2015

### Titania

I got it~ Thanks~

7. Aug 13, 2015

### athosanian

I dont 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.

8. Aug 13, 2015

### CWatters

9. Aug 13, 2015

### 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.

10. Aug 13, 2015

### Staff: Mentor

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

Last edited: Aug 13, 2015
11. Aug 13, 2015

### 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.

12. Aug 13, 2015

### phinds

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

13. Aug 13, 2015

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

14. Aug 13, 2015

### Staff: Mentor

15. Aug 15, 2015

### Alec Dacyczyn

*Cue mid-90s hip-hop references*

16. Aug 16, 2015

### sophiecentaur

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

17. Aug 19, 2015

### 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.

18. Aug 19, 2015

### phinds

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.

19. Aug 19, 2015

### jackwhirl

As demonstrated in SmarterEveryDay 85.
That too!

20. Aug 19, 2015

### stevmg

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.

21. Aug 19, 2015

### phinds

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?

22. Aug 19, 2015

### jackwhirl

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.

23. Aug 19, 2015

### Staff: Mentor

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

If you think otherwise, please provide a specific example.

24. Aug 19, 2015

### 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.

25. Aug 19, 2015

### phinds

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