Continuity Equation?

[DocDrakeRamoray]

Advertisement - Members don't see this ad

Hi guys, this my first post on this forum. I need some help with the contiuity equation.

As I understand increasing Area, velocity will go down.

But what if I have two funnels one's has cross-sectional area on the bottom is twice that of another funnel. How would the speed of fluid coming out of the funnel be different?


Thank you.

 

[chessknt]

Your first post is in the wrong forum. This isnt student doctor/homework help forum.

Type physics help forum on groogrles.

 

[funkydrmonkey]

Your first post is in the wrong forum. This isnt student doctor/homework help forum.

Type physics help forum on groogrles.

 

[DocDrakeRamoray]

Your first post is in the wrong forum. This isnt student doctor/homework help forum.

Type physics help forum on groogrles.

I thought this i where people ask questions regarding MCAT, No?

 

[funkydrmonkey]

I thought this i where people ask questions regarding MCAT, No?

http://forums.studentdoctor.net/forumdisplay.php?f=31
Mcat forum ^
Mcat forum I
Mcat forum I
Mcat forum I
Mcat forum I

 

[DocDrakeRamoray]

http://forums.studentdoctor.net/forumdisplay.php?f=31
Mcat forum ^
Mcat forum I
Mcat forum I
Mcat forum I
Mcat forum I

thank you much. I wasn't aware there is mcat forum. Thanks.

 

[WannaBePreMed]

this is 2 seconds away from being closed

 

[BlueElmo]

^^

Which is why I am going to post.

 

[BlueElmo]

And again.😛

 

[135892]

I don't understand the question...

Are you saying that you have two funnels, both with same cross sectional area on the top side, and then on the bottom side, one funnel (funnel A) has a cross sectional area of 2 times the the area of the other funnel (funnel B)? In this case, the velocity of the fluid coming out of funnel B would be twice the velocity of the fluid coming out of funnel A.

 

[rocketbooster]

I don't understand the question...

Are you saying that you have two funnels, both with same cross sectional area on the top side, and then on the bottom side, one funnel (funnel A) has a cross sectional area of 2 times the the area of the other funnel (funnel B)? In this case, the velocity of the fluid coming out of funnel B would be twice the velocity of the fluid coming out of funnel A.

yea, seriously. is this a troll? he/she wrote the answer directly above the question.

uhhhhh

 

[DocDrakeRamoray]

I don't understand the question...

Are you saying that you have two funnels, both with same cross sectional area on the top side, and then on the bottom side, one funnel (funnel A) has a cross sectional area of 2 times the the area of the other funnel (funnel B)? In this case, the velocity of the fluid coming out of funnel B would be twice the velocity of the fluid coming out of funnel A.

Hi, that's what I thought, but according to EK , the velocity is the same 😕. They used this formula to justify their answer v= square root of 2gh. Since there is no area in the formula the velocity of the fluid coming out of these two funnels would be the same. I used continuity equition to find the velocity, but apparently it is wrong.

 

[135892]

Hi, that's what I thought, but according to EK , the velocity is the same 😕. They used this formula to justify their answer v= square root of 2gh. Since there is no area in the formula the velocity of the fluid coming out of these two funnels would be the same. I used continuity equition to find the velocity, but apparently it is wrong.

I'd have to see the actual question then

 

[DocDrakeRamoray]

I don't understand the question...

Are you saying that you have two funnels, both with same cross sectional area on the top side, and then on the bottom side, one funnel (funnel A) has a cross sectional area of 2 times the the area of the other funnel (funnel B)? In this case, the velocity of the fluid coming out of funnel B would be twice the velocity of the fluid coming out of funnel A.

Sorry I wasn't clear with my question. But Basically this is the question ( you phrased it almost exactly the same way) . They even depicted them one with twice the cross-sectional area on the bottom side. But both have the same cross-sectional areas on the top side.

 

[Revilla]

Sorry I wasn't clear with my question. But Basically this is the question ( you phrased it almost exactly the same way) . They even depicted them one with twice the cross-sectional area on the bottom side. But both have the same cross-sectional areas on the top side.

I think there might be some phrasing nuance we're missing. Can you post the question as is from your EK book and give us the multiple choice answers? The MCAT forum is wonderful for helping with these kinds of questions.

 

[WannaBePreMed]

How is this thread still alive.

 

[TheMagicCookie]

How is this thread still alive.

My thoughts exactly...even in the MCAT forum there are specific rules against posting actually questions...

 

[Revilla]

My thoughts exactly...even in the MCAT forum there are specific rules against posting actually questions...

No there aren't. People post actual questions all the time. The only thing against the rules is posting questions from the actual MCAT. These are review materials and acceptable.

 

[Snarkalark]

Did you maybe read the question wrong? They might have mentioned difference in radius to trip you up.

 

[TheMagicCookie]

No there aren't. People post actual questions all the time.

"Please no actual MCAT questions."

copied and pasted from the mcat practice forum.

 

[Revilla]

"Please no actual MCAT questions."

copied and pasted from the mcat practice forum.

"Actual MCAT questions" means questions from the actual exam. This isn't a question from the "actual MCAT." It's a question from a review book. Completely acceptable.

 

[TheMagicCookie]

gotcha 👍

 

[DocDrakeRamoray]

This is from EK simulated exam 2f

Question: Funnels X and Y are filled with an ideal fluid to the same height. The Cross-sectional area of funnel Y at the bottom is twice that of funnel X at the bottom. Which of the following is TRUE concerning the funnels

A) The volume flow rate of the two funnels is the same

B) The velocities of the fluid coming out of the funnels are equal

C) The velocity of the fluid coming out of funnel Y twice the velocity of the fluid coming out of funnel X

D) The velocity of the fluid coming out of funnel X is twice the velocity of the fluid coming out of funnel Y.


I thought continuity equation should have been used. I was wrong. Can someone explain why the continity equation does not apply to this question. Thanks.

 

[starfishprime]

Hi, that's what I thought, but according to EK , the velocity is the same 😕.

It is important to understand that the MCAT is not always literally accurate. It is also important to understand that, when you write the AAMC a letter complaining of substantial ambiguity in several questions, they will almost certainly reject my appeal.

Convergent nozzles accelerate (subsonic) fluids, as anyone who owns both a garden hose and a thumb knows well. A 'funnel' with a small exit accelerates fluids relatively more than one with a large exit. In that case, your answer is correct.

However, there are two matters at hand.
1) They called it a 'funnel' and not a 'nozzle.'
2) The MCAT does not cover fluid dynamics.

So, then, pretend you're dropping marbles one at a time through your funnel. It's asinine but that's the name of the game.

 

[starfishprime]

Question: Funnels X and Y are filled with an ideal fluid to the same height. The Cross-sectional area of funnel Y at the bottom is twice that of funnel X at the bottom. Which of the following is TRUE concerning the funnels

A) The volume flow rate of the two funnels is the same

B) The velocities of the fluid coming out of the funnels are equal

C) The velocity of the fluid coming out of funnel Y twice the velocity of the fluid coming out of funnel X

D) The velocity of the fluid coming out of funnel X is twice the velocity of the fluid coming out of funnel Y.

Trivial. If Vx = 2Vy, D is true. If Vx = 2Vy, and 2Ax = Ay, Qx = VxAx = VyAy = Qy (which was your initial assumption). Since A and D cannot both be correct, and C is obviously wrong, that leaves B.

This is why the MCAT is a test of critical thinking.

 

[DocDrakeRamoray]

Trivial. If Vx = 2Vy, D is true. If Vx = 2Vy, and 2Ax = Ay, Qx = VxAx = VyAy = Qy (which was your initial assumption). Since A and D cannot both be correct, and C is obviously wrong, that leaves B.

thanks!

 

[Excelsius]

Trivial. If Vx = 2Vy, D is true. If Vx = 2Vy, and 2Ax = Ay, Qx = VxAx = VyAy = Qy (which was your initial assumption). Since A and D cannot both be correct, and C is obviously wrong, that leaves B.

This is why the MCAT is a test of critical thinking.

I think this was an interesting question because it is somewhat tricky. At first I went for D, but then realized we don't have pressure here. Logically, it ia possible to eliminate A and C right away. This leaves B and D. The question is what is the driving force here? It is gravity. We know that gravity is equal for all objects (g=9.8 m/s^2). Therefore, regardless of the size of the diameter, the liquid is going to FALL at the same rate. So an easy way to think about this problem would be to eliminate the funnel and even think of water droplets. Does a small water droplet fall as fast as a big one? Yes - neglect air friction. Therefore the answer is B.

Now if we were to add a pressure weight on top of the liquid simulating a syringe so that force of pressure > than the force of gravity, then the answer would be closer to D because P=F/A and Fx=Fy (same column of water) and Ax=Ay/2.

 

[starfishprime]

but then realized we don't have pressure here.

Of course we do. It's just V(d)g/A (and time-dependent).

 

[Excelsius]

Of course we do. It's just V(d)g/A (and time-dependent).

I meant other than the one caused by gravity. If you had no pressure at all, the fluid wouldn't flow (absent capillary action [before you try to correct me again])...

 

[rocketbooster]

Trivial. If Vx = 2Vy, D is true. If Vx = 2Vy, and 2Ax = Ay, Qx = VxAx = VyAy = Qy (which was your initial assumption). Since A and D cannot both be correct, and C is obviously wrong, that leaves B.

This is why the MCAT is a test of critical thinking.

exactly. must read all answer choices. tricky, tricky.

if A is true, D is true and vice versa. You can't have one true and one false since they must both be the same. since the question calls for one true answer, then A and D are both false haha. C is definitely wrong, so B by default.

I know you just wrote that out, but I wanted in on the action. I like this question a lot!

 

[Food]

This is actually one of the dumbest questions I've seen. I mean a question like this is designed, worded, and intended to be ambiguous to the point where you would have to follow an incredibly convoluted thought process to get the right answer. I'm pretty sure they throw questions like these out, when they do the scoring.