Physics concept I am having trouble grasping (Fluids in motion).

[JDoc9]

It is stated that decreased pressure creates increased velocity.
However, lets say we are increasing the area:
Per Constant=(A)(V), an increase in area would decrease the velocity.
then, per P=F/A, an increase area would decrease the pressure.
Therefore, those conditions decrease the pressure while decreasing the velocity, instead of the aforementioned phenomena of decreasing pressure creating an increase in velocity. What gives?

Thank you in advance for your help!

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[NextStepTutor_2]

It is stated that decreased pressure creates increased velocity.
However, lets say we are increasing the area:
Per Constant=(A)(V), an increase in area would decrease the velocity.
then, per P=F/A, an increase area would decrease the pressure.
Therefore, those conditions decrease the pressure while decreasing the velocity, instead of the aforementioned phenomena of decreasing pressure creating an increase in velocity. What gives?

Thank you in advance for your help!

Your confusion is understandable, and too many MCAT students simply memorize away without understanding. If your problem is with P=F/A "contradiction" you should realize there is no problem with it. The physics behind it is well beyond what is expected on the MCAT but what is likely the source of the confusion is your thinking is that you're not paying attention to the equation and realizing the force F changes too.

Let's recap what happens in your situation:

1. There's a change in cross-sectional area: A2 < A1
2. Thanks to conservation of mass, (1) implies v2 > v1
3. Thanks to Bernoulli, we know p2 < p1

Now take a look at this diagram

The dark blue rectangle on the left is what we call an element. Like the rest of the flow in the bigger section, it flows with velocity v1. v1 is delimited left and right by faces with area A1. Note that, since the liquid left and right of it has pressure p1, this element is compressed by forces F1=p1A1 on each side.

Now to the element on the smaller section, which flows faster. Its cross-sectional area is smaller. The pressure left and right of it is also smaller. As a result, the forces compressing it, F2=p2A2, are also smaller.

So, P =F/A still holds. Yes, when the situation changes, A is smaller, which by itself would make p bigger. However, as we saw above, then new F is smaller than the old one too, which by itself would make p smaller. The net effect of p2 < p1 (which we know beforehand from Bernoulli) means, therefore, simply that F'>F has diminished more than A'>A did.

This is still a simplification of what's truly happening (advection and diffusion of fluids)

Hope this helps, good luck!

 

[JDoc9]

Your confusion is understandable, and too many MCAT students simply memorize away without understanding. If your problem is with P=F/A "contradiction" you should realize there is no problem with it. The physics behind it is well beyond what is expected on the MCAT but what is likely the source of the confusion is your thinking is that you're not paying attention to the equation and realizing the force F changes too.

Let's recap what happens in your situation:

1. There's a change in cross-sectional area: A2 < A1
2. Thanks to conservation of mass, (1) implies v2 > v1
3. Thanks to Bernoulli, we know p2 < p1

Now take a look at this diagram

The dark blue rectangle on the left is what we call an element. Like the rest of the flow in the bigger section, it flows with velocity v1. v1 is delimited left and right by faces with area A1. Note that, since the liquid left and right of it has pressure p1, this element is compressed by forces F1=p1A1 on each side.

Now to the element on the smaller section, which flows faster. Its cross-sectional area is smaller. The pressure left and right of it is also smaller. As a result, the forces compressing it, F2=p2A2, are also smaller.

So, P =F/A still holds. Yes, when the situation changes, A is smaller, which by itself would make p bigger. However, as we saw above, then new F is smaller than the old one too, which by itself would make p smaller. The net effect of p2 < p1 (which we know beforehand from Bernoulli) means, therefore, simply that F'>F has diminished more than A'>A did.

This is still a simplification of what's truly happening (advection and diffusion of fluids)

Hope this helps, good luck!

Thank you so much for the prompt reply! I was going to go nuts today! Cheers to you.

So hypothetically, if a question asks what is happening to the velocity as area increases, its still a decrease in velocity, even though an increase in area corresponds to a decrease in pressure which corresponds to an increase in velocity???<-Nevermind. I answered my own question. The answer is no.