Posted by MarkB_MI on July 17, 2016 at 03:26:14 from (70.194.0.244):
In Reply to: Re: DOT?? posted by Geo-TH,In on July 16, 2016 at 18:10:48:
Good morning George!
1 slug = 32.174 pounds (mass) Acceleration of gravity on earth: 32.174 ft/s2
Coincidence? No. The mass unit of slug exists only so F=ma works out without using any conversion factors. Of course you just move the conversion out of the equation when you convert from pounds to slugs. In the real world nobody uses slugs. I don't buy a 644 slug sack of potatoes, I buy a 20 pound sack of potatoes. In the classroom, the slug is a useful unit of measure that clarifies the difference between weight and mass; in the real world it just gets in the way.
No, I don't have an MS in physics. My college physics was taught using only Systeme International (MKS) units, as were statics and dynamics. But the various thermodynamics courses I've taken used both SI and English units. Pounds-mass was always the unit of mass we used, NEVER slugs.
So, you ask, how do you get F=ma to work without using slugs as your unit of measure for mass? The engineer uses a conversion factor: one pound-force equals 32.174 pounds-mass feet/sec2. So to calculate the force required to accelerate 2000 pounds at 1 G:
F = ma = 2000 lbm x (1 lbf / (32.174 lbm-ft/s2)) x 32.174 ft/s2 = 2000 lbf
Notice that the pounds-mass units cancel out in this equation. As do the ft/s2 units. That's very important for the engineer; engineers always check unit consistency to confirm their calculations.
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