If a sneeze travels at 100mph, why can you stop it with a tissue?

  • Thread starter Thread starter dylansan
  • 16 comments
  • 2,953 views
dylansan

dylansan

Premium
Messages
5,046
United States
Massachusetts
Messages
GTP_dylansan
Messages
MINICOOPER120
IT's been bothering me. Is there some physics thing I don't get here? I'm sure there's others here who haven't thought of it, but now that they have, they're wondering too. So, does anyone smart want to tell me? Please? :)
 
force = mass * acceleration

Itty bitty mass —> itty bitty force.
 
Yep. It's just a little puff of wet air. Hardly any force. Plus tissues are porous, so the air can go through, leaving only a little bit of stuff needed to be stopped.
 
Surely if you held a tissue in a 100mph wind it would tear though, and air has even less mass than... snot.

Just holding a tissue on both sides and blowing on it fells like it puts a lot of force on it...
 
But it's a little volume of air. Think about it this way: Leaf blowers produce hurricane force winds, but they cannot knock over and blow away lawn furniture like a hurricane of the same wind speed (~70mph) can. It's a small volume with small impact area, so it emits a small force.
 
Hmmm, I suppose I didn;t think about it enough, but you'd still think that the small amount of air could tear through a tissue... Maybe it's time to ask the mythbusters?
 
Sage
force = mass * acceleration

Itty bitty mass —> itty bitty force.
Click to expand...

But 100 mph is a velocity ;) Maybe momentum = mass * velocity would be better suited here?

...

But actually what you need is Bernoulli's law:

static pressure + per-unit kinetic energy + per-unit gravitational potential energy = constant in laminar flow.

The tissue is close enough to your face to assume laminar flow.

Per unit gravitational potential energy is the same at your mouth and at the tissue, more or less, so we can drop that out of the equation. Also we can assume that the tissue stops the air completely, so the kinetic energy goes to zero.

So we have

pressure at your mouth + (1/2)(density of phlegm/air)*(100mph)^2 = pressure at the tissue

If we assume the pressure at your mouth is the same as the pressure on the back side of the tissue (means that you cheeks don't puff out when you sneeze; we can guess that the puffing is minimal ;)), then:

force on tissue = (1/2)(density of phlegm/air)*(100mph)^2*(surface are of tissue)

Then figure out if a tissue can hold that force, and you have your answer!
 
I never was good at physics. ;) I much prefer chemistry (except thermoeffingdynamics, which is basically physics anyway).
 
skip0110
But 100 mph is a velocity ;) Maybe momentum = mass * velocity would be better suited here?

...

But actually what you need is Bernoulli's law:

static pressure + per-unit kinetic energy + per-unit gravitational potential energy = constant in laminar flow.

The tissue is close enough to your face to assume laminar flow.

Per unit gravitational potential energy is the same at your mouth and at the tissue, more or less, so we can drop that out of the equation. Also we can assume that the tissue stops the air completely, so the kinetic energy goes to zero.

So we have

pressure at your mouth + (1/2)(density of phlegm/air)*(100mph)^2 = pressure at the tissue

If we assume the pressure at your mouth is the same as the pressure on the back side of the tissue (means that you cheeks don't puff out when you sneeze; we can guess that the puffing is minimal ;)), then:

force on tissue = (1/2)(density of phlegm/air)*(100mph)^2*(surface are of tissue)

Then figure out if a tissue can hold that force, and you have your answer!
Click to expand...


thats just what i was going to say.. :lol:
 
So, if a sneeze traveled at the speed of sound, would my boogers make a sonic boom?
 
MINICOOPER120
IT's been bothering me. Is there some physics thing I don't get here? I'm sure there's others here who haven't thought of it, but now that they have, they're wondering too. So, does anyone smart want to tell me? Please? :)
Click to expand...
100 mph = 160.9344 kph = 44.7m/s

Lets say the sneeze took 1/20th of a second (0.05s) to reach from your nose to the tissue. The gives it an acceleration of 44.7/0.05 = 894m/s^2. So now lets assume your sneeze weighed 10 milligrams (0.00001kg). The force exerted by a 10mg sneeze accelerating at 894m/s^2 is:

0.00001 * 894 = 0.00894N

Now, to put that in to perspective, that's about the same amount of force as a 1lb weight thrown at 5mph.

God I hate imperial units.

Edit: I could also rearrange the impulse formula to find the force of the sneeze.

F∆t = m∆v
F = mv/t = (0.00001*44.7)/0.05 = 0.00894N
 
They're not made of much, basically. There's not enough mass vs surface area to keep it travelling through the air at the same speed. If you fired a feather out of a gun (hypothetically of course. No chance of it working...) it wouldn't go as far as a bullet because the mass of it wouldn't be enough to counteract the air resistance. Same principle, I would have thought - have you ever actually felt a sneeze land? That's because it's made of... air, basically. Air hitting solid air won't do a lot. It's the equivilent of shooting a gun through 2ft of solid lead, I guess.

Why can you stop it with a tissue? The tissue has more mass, is stronger than the air. And the sneeze is slowed down a lot before it hits it. And you're actually holding the tissue, so part of it is you. If you left a tissue sitting on the top of your nose and you sneezed, it'd fly off.

Just my theory, really. There's probably something scientific in there.
 
Answering the OP question, for the same reason you need that stupid plastic tube on your can of WD-40. Once it's out of the restricted space of your throat and nose, it spreads and dissipates its speed. Becomes more like spray paint than WD-40.
 
Sneezing is quite a violent action in itself, to do the same at 650 odd MPH would be quite bad.

Also to answer the original question thread in a witty way...

Because it's too fast for you!
 

Latest Posts

Back