Can I suck myself forward with a straw when floating in air?

[Prishon]

I find myself in a space with air but no gravity. Say at ISS. Can I suck myself forward by sucking a straw? It reminds me somehow of Feynman's sprinkler.

Also Mach looked at something like Feynman's sprinkler. Mach invented something alike. Four ex/inhalers of air, tubes, that are in/exhale air in two perpendicular directions (in a plane). The tubes are connected to a ce tral reservoir. When air is pumped out the construction rotates but when it is sucked in nothing happens, at constant sucktion. I'm not sure what happens at accelerated sucktion.

 

[mfb]

No. The air will come from all directions evenly, providing no or essentially no force. Blowing out air will have a small effect, however.

 

[Prishon]

No. The air will come from all directions evenly, providing no or essentially no force. Blowing out air will have a small effect, however.

But inside the straw all air has one direction

 

[mfb]

Which stops once it reaches your body.

The overall system is (you) -> (you+air) with the air rushing in from all sides equally. You don't get net momentum because there is no net momentum anywhere else. Internal forces between you and the straw don't matter.

 

[Prishon]

Which stops once it reaches your body.

The overall system is (you) -> (you+air) with the air rushing in from all sides equally. You don't get net momentum because there is no net momentum anywhere else. Internal forces between you and the straw don't matter.

But most will come rushing in from the front of the straw. Like a vacuum cleaner.

 

[256bits]

But most will come rushing in from the front of the straw. Like a vacuum cleaner.

Right, you would be interested only in the change in momentum in the x-direction. The y- and the z-directions would cancel out due to symmetry.

Even so, where is the velocity change happening for the air in the x-direction.
How is that affecting you as you suck in air.

 

[Prishon]

Right, you would be interested only in the change in momentum in the x-direction. The y- and the z-directions would cancel out due to symmetry.

Even so, where is the velocity change happening for the air in the x-direction.
How is that affecting you as you suck in air.

The air is stopped inside your lungs. If it hits you down in the lungs you should go down. There is no change in horizontal momentum, I guess. The air of the room is sucked in, causing a permanent flow inward. Assuming you can inhale forever. Then it hits you inside.

 

[jbriggs444]

Assuming you can inhale forever. Then it hits you inside.

Wait, which is it? Can you inhale forever? Or does the inhaled air hit the bottom of your lungs and come to a stop?

 

[Prishon]

Wait, which is it? Can you inhale forever? Or does the inhaled air hit the bottom of your lungs and come to a stop?

You can inhale forever. This situation is so different from blowing air out. It is not the reverse.

 

[jbriggs444]

You can inhale forever. This situation is so different from blowing air out. It is not the reverse.

But you cannot inhale forever.

If you could, it would be the same thing as expelling the air out the back (you inhale but do not interact further with the inhaled air). So adopt that model instead if you like. Much simpler to form an intuition about.

 

[Prishon]

See also Feynman sprinkler

 

[Prishon]

But you cannot inhale forever.

If you could, it would be the same thing as expelling the air out the back (you inhale but do not interact further with the inhaled air). So adopt that model instead if you like. Much simpler to form an intuition about.

But you can imagine I could inhale forever.

 

[jbriggs444]

But you can imagine I could inhale forever.

I can imagine a number of possibilities, not all of which are equivalent.

One possibility is that the lungs inflate rapidly enough that the inhaled air does not further interact with the lungs at all. In which case it counts the same as if it had been expelled.

Another possibility is that the lungs expand laterally. The air comes to a stop against the back side and then expands perpendicular to the path along with it is inhaled.

Another possibility is that the lungs expand slowly. The air comes to an approximate stop.

 

[mfb]

But most will come rushing in from the front of the straw. Like a vacuum cleaner.

No, it comes from all sides, as explained before.

 

[jartsa]

I find myself in a space with air but no gravity. Say at ISS. Can I suck myself forward by sucking a straw? It reminds me somehow of Feynman's sprinkler.

Also Mach looked at something like Feynman's sprinkler. Mach invented something alike. Four ex/inhalers of air, tubes, that are in/exhale air in two perpendicular directions (in a plane). The tubes are connected to a ce tral reservoir. When air is pumped out the construction rotates but when it is sucked in nothing happens, at constant sucktion. I'm not sure what happens at accelerated sucktion.

Center of mass of the air changes a tiny bit when you inhale. Center of mass of the whole space station and its contents can not change. So I conclude that you can displace yourself a little bit by displacing air by inhaling. A long straw makes aforementioned process more efficient.

 

[.Scott]

No. The air will come from all directions evenly, providing no or essentially no force. Blowing out air will have a small effect, however.

@mfb is completely accurate, but let me further address this.

If you have never inflated a balloon without tying off the end - and then allowed it to fly off, you have had a deprived childhood. The rapidly deflating balloon acts like a small unguided rocket and rapidly visits its favorite corner(s) of the room.

So you might ask - why can't I run that experiment in reverse? It's because in the forward direction, the exiting air has momentum which keeps it going in the same direction. In reverse, the air approaching the balloon inlet will not be influenced by the direction it will follow once it reaches the balloon. There is no "anticipation momentum" tied to future events.

@jartsa 's statement "A long straw makes aforementioned process more efficient" relies on this non-existent "anticipation momentum".

 

[jartsa]

@mfb is completely accurate, but let me further address this.

If you have never inflated a balloon without tying off the end - and then allowed it to fly off, you have had a deprived childhood. The rapidly deflating balloon acts like a small unguided rocket and rapidly visits its favorite corner(s) of the room.

So you might ask - why can't I run that experiment in reverse? It's because in the forward direction, the exiting air has momentum which keeps it going in the same direction. In reverse, the air approaching the balloon inlet will not be influenced by the direction it will follow once it reaches the balloon. There is no "anticipation momentum" tied to future events.

@jartsa 's statement "A long straw makes aforementioned process more efficient" relies on this non-existent "anticipation momentum".

Let's say you sit on a skateboard holding a string that is tied to a big plastic bag filled with air, the bag is at the other side of the room. Now when you pull the string, the air in the bag moves towards you and you move towards the air. Then you and the air collide and stop. You are now at different spot than before.

Let's say you sit on a skateboard holding a tube that leads to a big plastic bag filled with air, the bag is at the other side of the room. Now when you suck air through the tube, the air in the bag moves towards you and you move towards the air. Then you and the air collide and stop. You are now at different spot than before.

 

[.Scott]

Let's say you sit on a skateboard holding a string that is tied to a big plastic bag filled with air, the bag is at the other side of the room. Now when you pull the string, the air in the bag moves towards you and you move towards the air. Then you and the air collide and stop. You are now at different spot than before.

Yes. That would be like pulling a parachute towards you.
You may not come to a complete stop when you reach the bag. You have created currents in the room that may have allowed you to exceed the speed of the bag - even if the bag was lighter than you.

Let's say you sit on a skateboard holding a tube that leads to a big plastic bag filled with air, the bag is at the other side of the room. Now when you suck air through the tube, the air in the bag moves towards you and you move towards the air. Then you and the air collide and stop. You are now at different spot than before.

The air is immediately colliding with the back of your throat.
Meanwhile:
1) The bag in collapsing and air in the room is moving towards it.
2) Your diaphragm is expanding and pushing air in the room away from you.

All of this could result in some minor change in your position - but not necessarily towards the bag.To exaggerate the effects, consider a massless sphere containing nothing - a vacuum.
We will place this shere in a weightless environment - and puncture it.

But before we puncture it, we will notice that there is atmospheric pressure applied uniformly to the outside surface of the sphere - and the sphere is not moving.

When we create a small puncture hole, there will now be one spot on the sphere that is no longer being pushed - so the sphere will start to move in that direction. Almost immediately (limited by the speed of sound), the air entering the sphere will reach the other side and any force that would have been applied to the puncture hole will now be applied to that inside surface. So the acceleration will be very short-lived.

Predicting the final result would be difficult. When the sphere suddenly jerks forward, it will be moving outside air with it - and even though the final speed of the sphere would normally be zero, it might leave drafts that could carry it for a while.

I suppose sticking it with a massless straw would have an effect. It does two things: 1) moves the hole to a different location - which in and of itself does not help; and 2) delays the amount of time between the creation of the hole and the impact of the air against the inside of the sphere, which would exaggerate the effect.

I think that what is important to realize is that forces that move you when you suck a straw are very minor compared to those that you get when you blow through a straw. That punctured sphere is not going to fly across the room like a opened toy balloon. It's going to jerk a bit, perhaps end up in a slightly different position, and perhaps end up drifting in whatever slight drafts it has created.

 

[A.T.]

See also Feynman sprinkler

Until ten years ago the explanation of the reversed sprinkler rotation relied on some air being accelerated, but not entering the pipe. This would allow for some propulsion in the case discussed in this thread. But it looks like meanwhile they found out that the air is simply spinning inside the vertical pipe, so the sprinkler spins the other way:

https://sciencedemonstrations.fas.h...nstrations/files/reverse_sprinkler.pdf#page=2

 

[sophiecentaur]

The directivity of 'blow', compared with the lack of directivity of 'suck' can be well demonstrated with a low frequency loudspeaker at high volume. This video is worth watching (just after 2 minutes is good) but I apologies for the mindless chat of the presenter.
You can do it yourself in front of your own HiFi.