Why divide by 1.059 to convert from slugs*ft/(s^2) to lbf?

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SUMMARY

The discussion centers on the conversion of force units from slugs*ft/(s^2) to pounds-force (lbf) using the factor 1.059 (lb*s^2)/(slug*ft). The original calculation yielded 9714.806 slugs*ft/(s^2), which is correct according to the solutions manual. The division by 1.059 is necessary to convert the units properly, as 1 lbf is defined as 1 slug*ft/s^2. The factor 1.059 is likely derived from the relationship between these units, ensuring accurate conversion in the context of the problem involving reverse thrust in aircraft.

PREREQUISITES
  • Understanding of English units of measurement, specifically slugs and pounds-force.
  • Familiarity with Newton's second law of motion (F = ma).
  • Basic knowledge of fluid dynamics and thrust calculations.
  • Ability to perform unit conversions in physics problems.
NEXT STEPS
  • Study the principles of unit conversion in physics, focusing on mass and force units.
  • Learn about the concept of thrust in fluid dynamics, particularly in relation to jet engines.
  • Explore the relationship between slugs, pounds-force, and gravitational acceleration (g = 32.174 ft/s^2).
  • Investigate the derivation and application of conversion factors in engineering problems.
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Students studying fluid mechanics, aerospace engineers, and anyone involved in physics or engineering calculations related to force and thrust in English units.

Toddy
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Homework Statement



Hello guys,

I just have a very quick question about English units. On my fluids homework, I got the answer 9714.806 slugs*ft/(s^2) which seems to be right according to the solutions manual. The homework problem is asking for the force , and the solution manual is dividing 9714.806 slugs*ft/(s^2) by 1.059 (lb*s^2)/(slug*ft) to get an answer of 9714 lb. Why is this right? I thought Slugs*ft/(s^2) = lbf.

Thank you for the help.
 
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Toddy said:

Homework Statement



Hello guys,

I just have a very quick question about English units. On my fluids homework, I got the answer 9714.806 slugs*ft/(s^2) which seems to be right according to the solutions manual. The homework problem is asking for the force , and the solution manual is dividing 9714.806 slugs*ft/(s^2) by 1.059 (lb*s^2)/(slug*ft) to get an answer of 9714 lb. Why is this right? I thought Slugs*ft/(s^2) = lbf.

Thank you for the help.
We really can't answer all your questions unless you post your HW.

1 lbf = 1 slug-s2 / ft

In more familiar F = ma terms, slugs = lbf / g, and g = 32.174 ft/s2

I don't recognize the factor 1.059 lb-s2 / (slug-ft), unless it's some weird density unit.

https://en.wikipedia.org/wiki/Slug_(mass)
 
SteamKing said:
We really can't answer all your questions unless you post your HW.

1 lbf = 1 slug-s2 / ft

In more familiar F = ma terms, slugs = lbf / g, and g = 32.174 ft/s2

I don't recognize the factor 1.059 lb-s2 / (slug-ft), unless it's some weird density unit.

https://en.wikipedia.org/wiki/Slug_(mass)

Hello Steamking,

Thanks for the reply,

The homework problem is the following.

Upon landing, airplanes exhaust the air from the engine through a reverse thrust mechanism in order to slow the plane. In this configuration, air flows into the jet engine shown at a rate of 9 slugs/s and a speed of 300 ft/s. Upon landing, the engine exhaust exits through the reverse thrust mechanism with a speed of in the direction indicated. Determine the reverse thrust applied by the engine to the airplane. Assume that the inlet and exit pressures are atmospheric and that the mass flow rate of fuel is negligible compared to the air flow rate through the engine

I was able to derived the equation for the reverse thrust :

300*9 + 900*cos(30)*4.5 + 900*cos(30)*4.5 = 9714 slugs*ft/(s^2)

Which seems to be right according to the solutions manual . However, I don't understand why they are dividing 9714 slugs*ft/(s^2) by 1.059 (lb*s^2)/(slug*ft) .

Any thoughts?

Thank you.
 

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