Pressure in Mouth: Calculate Using ρgh

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In summary, there was a question on a quiz about the pressure in one's mouth while holding liquid up by sucking or blowing. After some thought, it was determined that the pressure would be atmospheric pressure plus the change in pressure necessary to bring up the fluid 25cm, or in the case of blowing, atmospheric pressure minus the change in pressure. The 25cm measurement should be taken from the new fluid level, not the original level.
  • #1
johnhuntsman
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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 [itex]2/3[/itex] 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 [itex]2/3[/itex] 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.
 
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  • #2
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
haruspex said:
What is the pressure in your mouth when it is open?

Shouldn't it be atmospheric pressure?

haruspex said:
When you are holding the liquid up by sucking, is the pressure more or less, and by how much?

I would imagine there'd be more.

I suppose p = patm + ρgh should've been the answer then?
 
  • #4
johnhuntsman said:
I would imagine there'd be more.
Hmmm. So would it be less if you were to blow instead?
 
  • #5
haruspex said:
Hmmm. So would it be less if you were to blow instead?

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
Yes.

Provided the 25cm is measured from the new fluid level in the glass not the original level.
 
  • #7
CWatters said:
Yes.

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

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

1. What exactly is "pressure in mouth"?

Pressure in mouth refers to the force per unit area that is exerted on the inside of the mouth. This pressure is typically caused by the weight of a fluid (such as saliva) in the mouth and can also be influenced by external forces, such as the force of gravity.

2. How is pressure in the mouth calculated using ρgh?

The equation for pressure in the mouth is P = ρgh, where P is the pressure, ρ is the density of the fluid, g is the acceleration due to gravity, and h is the height of the fluid column. This equation is based on the principles of fluid dynamics and can be used to calculate the pressure at any point in the mouth.

3. What is the role of density in calculating pressure in the mouth?

Density plays a crucial role in calculating pressure in the mouth because it determines the amount of mass that is present in a certain volume of fluid. The higher the density, the more mass there is in the fluid, which results in a higher pressure. This is why denser fluids, such as saliva, can create a greater pressure in the mouth compared to less dense fluids, such as water.

4. How does the height of the fluid column affect pressure in the mouth?

The height of the fluid column is directly proportional to the pressure in the mouth. This means that the higher the fluid column, the greater the pressure will be. This is because the weight of the fluid increases with height, resulting in a greater force pushing down on the bottom of the column and creating a higher pressure.

5. Can pressure in the mouth be influenced by external forces?

Yes, pressure in the mouth can be influenced by external forces, such as the force of gravity. For example, if a person is standing upright, gravity will pull the fluid downwards, causing a higher pressure at the bottom of the mouth compared to the top. This is why we often feel a greater pressure in our mouths when we are standing compared to when we are lying down.

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