When we sneeze, our head moves forward. However since the air from our mouth moves forward with a high velocity, shouldn't the head should recoil backwards (like a gun, to conserve the momentum) instead of moving forward?
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The sneeze is moving at about 4.5 m/s, and the total volume is likely less than a litre (typical human lung capacity is about 6 litres). So the total mass expelled will be less than one gram, and the momentum will be less than $p_{sneeze}=4.5\times 10^{-3}$ kg m/s.
An adult human head masses about 4.5 kg. By momentum conservation, $p_{sneeze}+p_{head}=0$, or $v_{head}=p_{sneeze}/m_{head}$. In this case, that is $v_{head}\approx 10^{-3}$ m/s. So the recoil is tiny, easily balanced by neck muscles.
answered Jul 19, 2018 at 5:03
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2$\begingroup$ Probably because of some reflex - the effect is purely muscular. $\endgroup$ Commented Jul 19, 2018 at 5:26