Fluids, finding the density of a sphere in oil Please help

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SUMMARY

The discussion focuses on calculating the drag force and density of a small sphere (diameter = 10 mm) falling in SAE 10W oil at 200 degrees Celsius with a constant velocity of 50 mm/s. The drag force was calculated using the equation F = Cd * 0.5 * ρ * A * U², resulting in a value of 225 N. However, the participant encountered difficulties in determining the sphere's density, as equating the drag force to the weight yielded incorrect results. The correct application of the drag force equation is crucial for accurate density calculation.

PREREQUISITES
  • Understanding of fluid dynamics principles, specifically drag force calculations.
  • Familiarity with the drag coefficient equation, Cd = 24/Re^0.645.
  • Knowledge of basic physics concepts such as weight and buoyancy.
  • Proficiency in unit conversions, particularly between mm and meters.
NEXT STEPS
  • Review the derivation and application of the drag coefficient equation in fluid dynamics.
  • Study the relationship between drag force and weight in falling objects.
  • Learn about the properties of SAE 10W oil and its impact on drag calculations.
  • Explore methods for calculating density from drag force and volume.
USEFUL FOR

Students in physics or engineering courses, particularly those studying fluid dynamics, as well as professionals involved in materials science and mechanical engineering who require knowledge of drag forces and density calculations in fluids.

abs123456
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Homework Statement



A small sphere (diameter = 10 mm) is left to fall in a fluid (oil, SAEIOW) at 200 degrees
with a constant velocity of 50 mm/s..

The Following data is available:
- For SAEIOW oil at 20 degrees: p = 920 kg/m3, μ = 0.1 Pa.s

a) The drag force on the sphere
b) The density of the sphere.



Homework Equations




Cd = 24/Re^0.645

The Attempt at a Solution



a) I managed to do the first part, by using Cs in the equation F= Cd/0.5 x rho x U^2 x A

And i got 225N..

b) However i am not too sure about doing part b... if i equate the drag force to the weight it gives me
the wrong answer for density.. This part is a 13 mark question too..
 
Physics news on Phys.org
I think you have an error in the drag force equation.
F = Cd * 0.5 * rho * A * U^2
 

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