Fluid Mechanics -Momentum Equation

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SUMMARY

The discussion centers on the application of the momentum equation in fluid mechanics to determine the scale reading of a weigh tank after water enters it. The initial calculation yielded a weight of 74.8 lbf, but the correct reading after 10 seconds is 212.67 lbf. The discrepancy arose from a miscalculation regarding the weight of the water and the omission of the gravitational constant (gc) factor. The momentum change of the water, which discharges vertically downward, must be accounted for to accurately assess the total weight on the scale.

PREREQUISITES
  • Understanding of fluid mechanics principles, specifically the momentum equation.
  • Familiarity with gravitational constant (gc) in weight calculations.
  • Knowledge of flow rate calculations through pipes, including diameter and velocity.
  • Basic skills in problem-solving and mathematical calculations related to physics.
NEXT STEPS
  • Study the application of the momentum equation in fluid dynamics.
  • Learn how to calculate flow rates using the continuity equation.
  • Research the role of gravitational constant (gc) in weight and force calculations.
  • Explore examples of weigh tank calibration and flow meter accuracy assessments.
USEFUL FOR

This discussion is beneficial for students and professionals in engineering, particularly those specializing in fluid mechanics, as well as anyone involved in the calibration of flow measurement systems.

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Fluid Mechanics --Momentum Equation

Homework Statement


A large weigh tank is to be used in the calibration of a flow metre. Measurements of weights as a function of time are to be made. Water enters the tank vertically from the flow metering system at a speed of 20 ft/s through a 1.5 in. diameter pipe. If the weight of the empty tank is 50 lbf, determine the scale reading at t=10 s.

I found the reading to be 74.8 lbf but the answer is 212.67 lbf.

Any help is appreciated.

The link to my working.

https://skydrive.live.com/?sc=photos&cid=6b041751c72e14ad#cid=6B041751C72E14AD&id=6B041751C72E14AD%21173
 
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I am unable to decipher your link. Without considering the change in momentum of the water, I get about 203 pounds. If the pipe discharges vertically downward, you'll have to consider momentum change of the water adding to the weight of the vessel.
 


Thanks for the reply.
I have already solved the question. Turned out there's a mistake when I calculated
the weight of water after 10 seconds and I did not include the gc factor. And yes,
momentum of the falling water is already included in the momentum equation.
 

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