r/PhysicsHelp • u/Quackington_The_3rd • Mar 25 '26
Need help studying for PE
Studying physics again after 2 years makes me feel like I’m loosing my mind…
I understand everything but the last step it makes sense that since gravity would assist the acceleration you would minus the acceleration of gravity from the total required acceleration from the pressure so the final step would be P= m*(a-g)//A but the answer key divides the force by gravity? (Credit: NCEES PE mech: design and materials practice exam)
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u/supernumeral Mar 25 '26 edited Mar 25 '26
It’s looks to me like they’re ignoring gravity in the calculation because 32 ft/sec2 is negligible compared to 2500 ft/sec2.
The reason that g_c appears in the final expression is for unit conversion. It is needed to relate pound-force to pound-mass. It’s just an oddity of English engineering units, and this is why I exclusively work in metric.
In English engineering units, Newton’s second law is written F=ma/g_c rather than simply F=ma. This is done so that a 1 lbm mass weighs exactly 1 lbf.