8
$\begingroup$

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.

$\endgroup$
  • 4
    $\begingroup$ 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 :-) $\endgroup$ – John Rennie May 8 '12 at 16:59
16
$\begingroup$

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.

$\endgroup$
  • $\begingroup$ Presumably "1.3mm/s" should be "1.3 m/s". $\endgroup$ – dmckee May 8 '12 at 17:19
  • $\begingroup$ No I think 1.3mm/s is correct. Maybe I should rewrite it as 0.0013m/s. $\endgroup$ – John Rennie May 8 '12 at 17:20
  • 1
    $\begingroup$ +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. $\endgroup$ – Nathaniel May 8 '12 at 19:38
  • 1
    $\begingroup$ @Pablo: as Nathaniel says, it's not the velocity of sneeze, all the sneeze does is redistribute the astronaut's mass and move the centre of mass relative to the astronaut's body. So the astronaut's centre of mass doesn't move, but his mouth moves relative to the position of the centre of mass. The 4 microns is a gross overestimate since, again as Nathaniel says, it's assuming the 0.6g air moves from the astronauts mouth to the faceplate, and of course all it does it generate some air currents. The calculation was not meant to be all that serious. $\endgroup$ – John Rennie May 9 '12 at 6:14
  • 1
    $\begingroup$ Current space suit designs operate at ~1/3rd to 1/5th atmospheric pressure; so your back of the envelope estimate's a bit on the high side. en.wikipedia.org/wiki/Space_suit#Operating_pressure $\endgroup$ – Dan Neely May 9 '12 at 13:21
2
$\begingroup$

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.

$\endgroup$
  • $\begingroup$ It's surely right $\endgroup$ – Zignd May 8 '12 at 18:09

Not the answer you're looking for? Browse other questions tagged or ask your own question.