Solved Problem: Linear Momentum Quiz

MEC516/BME516 Fluid Mechanics, Chapter 3: A short quiz problem that demonstrates how to obtain an expression for the forces in a pipe elbow using the principle of conservation of linear momentum.
A more detailed discussion of the general solution method (and the reason why gauge pressures must be used) is discussed in this video:
• Solved Problem: Conser...
All the videos in this course and a copy (pdf) of this presentation can be downloaded at: www.drdavidnaylor.net
Course Textbook: F.M. White and H. Xue, Fluid Mechanics, 9th Edition, McGraw-Hill, New York, 2021.
#fluidmechanics #linearmomentum

[@FluidMatters]

All the videos in this course, pdfs of the presentations, plus solved exams and problem sets can be downloaded at: www.drdavidnaylor.net

[@danielaloaiza6077]

Sir, you are the reason why I understand fluids, saludos desde Colombia❤

[@FluidMatters]

Glad to hear the videos are helpful. Best of luck with your studies.

[@jacobhardy2313]

Great video and explanation

[@FluidMatters]

Thanks. Glad to hear it was helpful.

[@MuhammadQasim-th3ed]

Sir please upload more video like that conceptual video ...please sir 💛👍 Thanks alot you are very great human being 💛😊👍💥🔥 ...

[@FluidMatters]

Glad to hear it was helpful.

[@Ganesh-ih6vv]

Sir continue this classes on KZbin Very helpful Thanks for your efforts sir

[@FluidMatters]

Glad to hear this video was helpful. Thanks for the kind words.

[@my_unreasonably_long_username]

So if I have a 1000m straight length of piping above the elbow, do I need to account for that in W, or is that already taken care of by P1*A1 with P1 being the hydrostatic pressure from the 1000m column of water above it given by rho*g*h?

[@FluidMatters]

You are correct. The weight of the fluid in the pipe above the elbow will be accounted for by the pressure at the flange. However if, for example, the 1000m pipe above the flange was totally unsupported, you would need to include the downward weight force (of the 1000m pipe) at the upper flange. So, you need to include known external forces on the control volume.

[@KT-dv8qy]

THANKS A LOT SIR!!!!

[@FluidMatters]

Glad to hear it was helpful. Best of luck with your studies.

[@nahorb]

very good video

[@FluidMatters]

Thanks.

[@user-mf7vr3sj4u]

Can you explain why (at 1:05) the pressure forces act inwards as opposed to in the direction of flow / where in Chapter 2 specifically should I look to find out why?

[@FluidMatters]

p. 61 of the White text: "Pressure creates a force due to the molecules bombarding the surface, and it is normal to the surface." Fluid molecules bouncing off a surface push on the surface. So, the pressure force acts inward and normal to the surface. Also, see the proof of that pressure is a point property in Chapter 2, Fig. 2.1. Equilibrium on a small wedge of fluid. Think also about your experience. A balloon of air submerged in deep in water will get compressed by the water pressure, because the static pressure acts inward. If pressure acted outward, it would expand, which it certainly does not. I hope that helps.

[@arundhati.parikh]

@@FluidMattersthank you! That makes complete sense for the inlet… however, at the outlet, aren’t the molecules moving away from the surface (in the direction of flow) as opposed to pushing against the surface? maybe I am missing something?

[@FluidMatters]

@@arundhati.parikh The bulk motion of fluid is not the root cause of static pressure. Maybe this will help: Do you expect the outflow side of elbow to be held onto the pipe at the flange by suction? That's what you are thinking, which I hope you can sense is clearly not what happens.