Hovercraft B/C

[nicholasmaurer]

So what what you consider to be the answer? I guess they are assuming the velocity is constant, but that is not clear. Also, with you hose example, the flow rate is constant. The area decreases but velocity increases, keeping a constant volumetric (or mass) flow rate.

Anyway, I put 4:1 because I had the same logic (that I now recognize is faulty), but got it wrong.

Huh, I would think that v(b) = 4 * v(a). Maybe they wanted you to specify that the velocity in pipe B was 4 times as fast as pipe A rather than just having 4:1 without specifying. Or I could just be wrong and the velocity is constant but why.

I would also agree it should be 4x, but the answer key says 16x. I have asked the supervisor to verify the answer and explain the math.

From the Event Supervisor: you must use the Poiseuille Equation, where radius is to the fourth power. Q = pi*(delta P)*r^4 / (8 * n * l). Since the only thing changing is the cross-sectional radius of the pipe and it is to the fourth power, then the difference is 16 times (2^4)

[dmis]

Huh, I would think that v(b) = 4 * v(a). Maybe they wanted you to specify that the velocity in pipe B was 4 times as fast as pipe A rather than just having 4:1 without specifying. Or I could just be wrong and the velocity is constant but why.

I would also agree it should be 4x, but the answer key says 16x. I have asked the supervisor to verify the answer and explain the math.

From the Event Supervisor: you must use the Poiseuille Equation, where radius is to the fourth power. Q = pi*(delta P)*r^4 / (8 * n * l). Since the only thing changing is the cross-sectional radius of the pipe and it is to the fourth power, then the difference is 16 times (2^4)

Thank you. Did he also provide a solution for the other problem?

[RestingDoll]

Huh, I would think that v(b) = 4 * v(a). Maybe they wanted you to specify that the velocity in pipe B was 4 times as fast as pipe A rather than just having 4:1 without specifying. Or I could just be wrong and the velocity is constant but why.

I would also agree it should be 4x, but the answer key says 16x. I have asked the supervisor to verify the answer and explain the math.

From the Event Supervisor: you must use the Poiseuille Equation, where radius is to the fourth power. Q = pi*(delta P)*r^4 / (8 * n * l). Since the only thing changing is the cross-sectional radius of the pipe and it is to the fourth power, then the difference is 16 times (2^4)

The issue is that nothing in the problem indicates that Poiseuille Equation should be applied (viscosity, laminar flow, etc).

[mattruff]

When we are about to test the hovercraft at competition, do the rolls of pennies go on after or before we start up the motors?

[nicholasmaurer]

When we are about to test the hovercraft at competition, do the rolls of pennies go on after or before we start up the motors?

I don't believe it matters. You just have to indicate when you are ready to run. As a practical matter, I would recommend before, as running your motors can quickly drain your batteries and throw off calibrations.

[reed303]

When we are about to test the hovercraft at competition, do the rolls of pennies go on after or before we start up the motors?

Before.
See Rule 4.l "To begin a run, a team will place their vehicle,including the penny load,on the track at the start line against the wood block placed by the supervisor. A team then activates their vehicle’s motor(s)."

[nicholasmaurer]

The issue is that nothing in the problem indicates that Poiseuille Equation should be applied (viscosity, laminar flow, etc).

I think that is a fair criticism. The ES agreed he should have given more information to indicate the Poiseuille equation was needed. I think his assumption was that most pipes meet the requirements of using Poiseuille: they are reasonably long and not so wide as to generate turbulent flow.

[geniusjohn5]

If I connected two 9v batteries in parallel, would that have any effect on the speed of the fan/motor? I'm trying to get my hovercraft to move forward but it is unable to

[geniusjohn5]

Also could using a hovercraft that is much smaller than the track width(like a hovercraft from last year) work for this year? Since it would just be hugging the rails of the track.

[dmis]

If I connected two 9v batteries in parallel, would that have any effect on the speed of the fan/motor? I'm trying to get my hovercraft to move forward but it is unable to

Yes, it could. 9v batteries have a high internal resistance, which will slow your motor. Paralleling them could help a bit. You would likely be better off with a different type of battery, even if it is a lower voltage.