Mike Armstrong wrote:
Like below?
Mike
This is dangerously WRONG. the unit "lb" is mass, equivalent to slugs, kilograms, etc, and a direct replacement.
The unit "lbf" is a force, equivalent to Newtons and Dynes (aand according to version 11, kilograms force kgf)
E, Young's Modulus has units of stress (psi) which is force per unit area. Creating it as above, dividing by gravity constant to create mass per unit area is not only wrong but dangerous since it introduces a basic unit balance error into the calculation.
You must recognise that there is a difference between lb and lbf--they represent two different things (mass and force). Get used to lbf. For E, as above, try psi--it works great and is as easy to type.
Fred
Mike Armstrong wrote:
I didn't think there would be a problem dividing by g though as long as it was noted and kept track off.Mike
The problem comes when you use Young's Modulus somewhere else, like in a deflection calculation. Would you like your deflection in meters or Sieverts?
Fred
The problem comes when you use Young's Modulus somewhere else, like in a deflection calculation.
You could just multiply the Young modulus by g........Only joking. 
Would you like your deflection in meters or Sieverts?
I would prefer my deflection in mm to be honest. When deflection is getting towards meters it's time to worry.
Mike
Mike Armstrong wrote:
Would you like your deflection in meters or Sieverts?
I would prefer my deflection in mm to be honest. When deflection is getting towards meters it's time to worry.
Mike
You may have you deflection in any length unit you prefer, it will still be the correct value. And yes, you could avoid the problem by multiplying by "g," thus correcting (locally) the error you generated to get lb instead of lbf.
One of the very nice features of Mathcad is unit checking. If the units you get in a result are not the units you think you should, then it's time to start looking closely at the values, units, and equations that got you there. Intentionally creating a booby-trap is just bad practice. (The late lamented JmG was a strong opponent of units in Mathcad. He would have advocated setting lbf = 1 and lb = 1; then lbf = lb and we're all good. This "discussion" topic generated some of the record-length chains in the old collaboratory. You've tripped over a strong proponent of the correct use of units. I'll climb down off the soapbox now!
)
. I'll climb down off the soapbox now!
You can stay on it if you like, but I think with jmG gone you are mostly preaching to the converted.
Of course, we could get into the slightly more controversial subjects of Hz vs cycles and whether or not angles should have units. We haven't generated a huge, long thread on one of those topics for at least a month 
You may have you deflection in any length unit you prefer, it will still be the correct value. And yes, you could avoid the problem by multiplying by "g," thus correcting (locally) the error you generated to get lb instead of lbf. One of the very nice features of Mathcad is unit checking. If the units you get in a result are not the units you think you should, then it's time to start looking closely at the values, units, and equations that got you there. Intentionally creating a booby-trap is just bad practice. (The late lamented JmG was a strong opponent of units in Mathcad. He would have advocated setting lbf = 1 and lb = 1; then lbf = lb and we're all good. This "discussion" topic generated some of the record-length chains in the old collaboratory. You've tripped over a strong proponent of the correct use of units. I'll climb down off the soapbox now!
Hope I haven't started another argument/ discussion about units again. BTW I wouldn't manipulate units like that, just showing it could be done, even if wrong.
Mike
