# Homework Help: Pressure in Mouth

1. Nov 28, 2012

### johnhuntsman

There was a question on a quiz I just took and I want to make sure I got it right.

Something like:
"A beaker filled with to $2/3$ its height with a fluid of density ρ = 1500kg / m3. You place a straw in the fluid such that the fluid in the straw is at a height equal to that of the fluid prior to having a straw put in (i.e., the height of the fluid is still $2/3$ of the max height). You suck the fluid in the straw up 25cm. What is the pressure in your mouth?"

After a good while of trying to overthink it (using Bernoulli's principle) I figured that it would just be ρgh = (1500kg / m3)(9.81m / s2)(0.25m). Is it right or wrong? It may very well be the difference between an A and a B so it'll just kill me to not know 'til I find out my grade Monday.

Last edited: Nov 28, 2012
2. Nov 28, 2012

### haruspex

What is the pressure in your mouth when it is open?
When you are holding the liquid up by sucking, is the pressure more or less, and by how much?

3. Nov 28, 2012

### johnhuntsman

Shouldn't it be atmospheric pressure?

I would imagine there'd be more.

I suppose p = patm + ρgh should've been the answer then?

4. Nov 28, 2012

### haruspex

Hmmm. So would it be less if you were to blow instead?

5. Nov 28, 2012

### johnhuntsman

Nevermind I suppose it'd be the other way around. Fluid would be sucked in (as air would be) because the pressure in the mouth is less. So that leaves me at a complete loss as far as what the answer is concerned.

Would it be patm - ρgh? ρgh being the change in pressure necessary to bring up that fluid 25cm?

6. Nov 29, 2012

### CWatters

Yes.

Provided the 25cm is measured from the new fluid level in the glass not the original level.

7. Nov 29, 2012

### johnhuntsman

My teacher did say not to take change in original fluid level into account, but I gotcha. Thanks, I appreciate it.