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Naive question; feel free to shoot me down

It is a truism that any motion in space would continue indefinitely unless it is opposed by an external force. If a cosmonaut were to sneeze within his/her spacesuit, would it have any impact upon their movement? I assume the suit and cosmonaut would, between them, totally absorb the force exerted by the 100mph sneeze air velocity. This would leave the cosmonaut unaffected ... probably.

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I think you meant a "truism" not an "altruism", though I suppose it is altruistic to climb into a spacesuit if you feel a cold coming on :-) –  John Rennie May 8 '12 at 16:59
@JohnRennie: Thank you (+: I updated it –  Everyone May 8 '12 at 17:06

2 Answers 2

up vote 13 down vote accepted

When the cosmonaut sneezed they would start moving, and rotating, in the opposite direction, but when the sneeze hit their faceplate (ugh!) this would stop the motion. The net result is that the velocity of the cosmonaut would not have changed, but their position and angle would have.

According to Wikipedia a typical breath is 500cm$^3$ and a sneeze velocity is around 15m/s. If the density of air is about 1.2kg/m$^{3}$ the momentum of a sneeze is about 0.009kg.m/s. I weight about 70kg, so the sneeze would leave me moving at about 0.0013m/s. Lets say the faceplate is 5cm away from my mouth, then with the sneeze moving at 15m/s I'd only move for 0.0033s before the sneeze hit my faceplate (ugh again!) and stopped me. In that time I'd have moved about 4 microns.

I must admit that's less than I thought when I started this.

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Presumably "1.3mm/s" should be "1.3 m/s". –  dmckee May 8 '12 at 17:19
No I think 1.3mm/s is correct. Maybe I should rewrite it as 0.0013m/s. –  John Rennie May 8 '12 at 17:20
Oh...sorry, I read the whole sentence wrong. Carry on. –  dmckee May 8 '12 at 17:22
Ern.My sneeze is distinctly faster ... and louder. I remember at-least one occasion when the reaction/jerk damaged the cubicle wall. That might be momentum though. –  Everyone May 8 '12 at 18:19
+1, but you don't need to use the velocity to do this calculation, since you know the centre of gravity will not move (or keep moving in a straight line). The air will displace other air, so it will just recirculate inside the suit and have no effect on the astronaut's position. So ignoring rotation we have distance moved by astronaut = distance moved by snot * mass of snot on faceplate (ugh) / mass of astronaut = not very much. –  Nathaniel May 8 '12 at 19:38

Cosmonaut would not budge because the force of the sneeze pushes him back and when the sneeze hits the helmet that makes a forward force, nullifying the two forces.

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It's surely right –  Zignd May 8 '12 at 18:09

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