Rusty, can someone check my f=ma math
Rusty, can someone check my f=ma math
(OP)
Guys:
Long time since I've had to do these simple calcs. Gettign old, I barely rememebr the details on accelleration due to gravity :). Can one of you confirm I'm not doing something dumb with my mass and weight conversions?
Problem: I have a weight that is 20lbs (bathroom scale). I am going to accelerate it horizontally, 3.24 ft/sec2 from rest. (assume no fracton)
I think I calculate the force to be
F=ma
F= (20 lbs /(32.17ft/sec2)) * 3.24 ft/sec2
F = 2.01 lbs(f)
That's right isn't it?
Thanks!
Scott
Long time since I've had to do these simple calcs. Gettign old, I barely rememebr the details on accelleration due to gravity :). Can one of you confirm I'm not doing something dumb with my mass and weight conversions?
Problem: I have a weight that is 20lbs (bathroom scale). I am going to accelerate it horizontally, 3.24 ft/sec2 from rest. (assume no fracton)
I think I calculate the force to be
F=ma
F= (20 lbs /(32.17ft/sec2)) * 3.24 ft/sec2
F = 2.01 lbs(f)
That's right isn't it?
Thanks!
Scott





RE: Rusty, can someone check my f=ma math
Yes looks right to me
desertfox
RE: Rusty, can someone check my f=ma math
A.
RE: Rusty, can someone check my f=ma math
RE: Rusty, can someone check my f=ma math
You guys are funny :).
re scale, I just meant to point out I was using weight and not mass, as might be measured with a normal earth bound scale. :). Just needed to be sure I rememebred the 32.17ft/sec2 divisor correctly. Been 25 years since I had to use it in class. ME degree, but ended up in a more EE/IT vocation.
What I really have is a linear bearing with a 20lb load who's CG is sitting about 8" off the rail, and gets moved by a step motor. I needed to calculate the torque that happens when I accellerate the truck (and load).
Thanks!
Scott
RE: Rusty, can someone check my f=ma math
TTFN
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RE: Rusty, can someone check my f=ma math
Thanks guys!
RE: Rusty, can someone check my f=ma math
RE: Rusty, can someone check my f=ma math
RE: Rusty, can someone check my f=ma math
- Steve
RE: Rusty, can someone check my f=ma math
I find this *safer* than SI, where g and gc are not numerically equal, and a failure to convert between kg and N puts you off by a factor of 10.
RE: Rusty, can someone check my f=ma math
RE: Rusty, can someone check my f=ma math
The OP understands the difference between weight and mass. This makes the SI equations work perfectly with English units, and convert easily to metric units, as long as you understand that pounds convert to Newtons. If you think in terms of pounds mass, as noted above, conversions to and from metric will be weird and exciting, and you will have to keep track of two systems of equations.
RE: Rusty, can someone check my f=ma math
the other way to see the answer is his acceleration is g/10 (not /3, oops) so the force is 20/10 = 2 lbs.
sorry for the obfuscation.
RE: Rusty, can someone check my f=ma math
Then again, from past experience, I don't expect anything I or anyone else says on this forum will affect anyone else's approach to units one iota. Here's hoping that my contribution is not one in a long series of posts on SI vs non-SI.
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(2B)+(2B)' ?
RE: Rusty, can someone check my f=ma math
RE: Rusty, can someone check my f=ma math
Tobalcane
"If you avoid failure, you also avoid success."
"Luck is where preparation meets opportunity"
RE: Rusty, can someone check my f=ma math
I do use charts for lbs-f, psi, ksi, lbs-ft and such when using fora like this one, because I don't necessarily always have an idea of the order of magnitude of your system.
RE: Rusty, can someone check my f=ma math
I remember just converting to kg/N in college too. Remembered it simplyfing it somewhat. But just wanted to be sure I had the englich way right. Wit or not :)
RE: Rusty, can someone check my f=ma math
Tobalcane
"If you avoid failure, you also avoid success."
"Luck is where preparation meets opportunity"
RE: Rusty, can someone check my f=ma math
RE: Rusty, can someone check my f=ma math
m=w/g.
f=ma = (w/g)a
Pounds are a unit of force, convertible to Newtons. You can ignore slugs, otherwise, they convert to kilograms somehow. I cannot be bothered to look it up.
Now, you have one system of equations to memorize.
Pounds or ounces, feet, inches or furlongs, and seconds, minutes etc., all are basic units. You can convert them to your heart's content. Newtons are derived from meters and seconds. If you use Newtons, your length units are meters, and your time units are seconds. Your results could be weird and inaccurate if you try anything else.
I must admit, when I set up a spreadsheet with calculations, I methodically convert everything to the SI meters, kilograms and seconds (MKS).
RE: Rusty, can someone check my f=ma math
RE: Rusty, can someone check my f=ma math
TTFN
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RE: Rusty, can someone check my f=ma math
My little spring scale is marked in N and g, when it ought to be N and lb. I am sure lots of mass balances are marked in lb. This is harmless until you do calculations, especially if we stay here on planet earth.
RE: Rusty, can someone check my f=ma math
RE: Rusty, can someone check my f=ma math
RE: Rusty, can someone check my f=ma math
I do not approve of that either, then again, how about converting N.m torque into Joules?
thread1183-295198: Bending moment calculation
RE: Rusty, can someone check my f=ma math
Tobalcane
"If you avoid failure, you also avoid success."
"Luck is where preparation meets opportunity"
RE: Rusty, can someone check my f=ma math
1 lbf = 1 slug*1 ft/sec^2 or
1 lbf = 1 lbm*32.17 ft/sec^2
RE: Rusty, can someone check my f=ma math
Fe
RE: Rusty, can someone check my f=ma math
Fe
RE: Rusty, can someone check my f=ma math
For mass moment of inertia in English units, substitue w/g for m. Use the SI equations. If you ignore slugs, you can use inches instead of feet as your length units.
lb/(in/sec2) × in2 = lb.in.sec2.
This looks a little weird, but if you convert this unit to MKS, you get...
N.m.s2 = kg.m/s2 × m.s2 = kg.m2.
RE: Rusty, can someone check my f=ma math
and i'd add the "f" to "lb"
RE: Rusty, can someone check my f=ma math
RE: Rusty, can someone check my f=ma math
Tobalcane
"If you avoid failure, you also avoid success."
"Luck is where preparation meets opportunity"
RE: Rusty, can someone check my f=ma math
1N = 1kg * 1m/sec^2
1kgf = 1kg * 9.8m/sec^2 = 9.8N
but ...
1lbf = 1lbm*32.17ft/sec^2 = 1 slug*1ft/sec^2
RE: Rusty, can someone check my f=ma math
Tobalcane
"If you avoid failure, you also avoid success."
"Luck is where preparation meets opportunity"
RE: Rusty, can someone check my f=ma math
lbm is STILL NOT weight. It just happens to be numerically equal to weight (lbf) at standard earth gravity. Lbm is mass, just as kg and slugs are mass.
In SI you must properly label your units. In English you must properly label your units. If you DO NOT label units, and/or insist on substituting one unit type for another, things will work out poorly no matter what system you decide to use.
RE: Rusty, can someone check my f=ma math
"So what do you get when you convert kg (which is mass) to lb or vice versa?"
because 1kgm has a weight of 1 kgf which is the same as 9.8N which equals 2.2 lbf (so a 1kg bag of sugar in the supermarket weights 2.2 lb).
RE: Rusty, can someone check my f=ma math
- Steve
RE: Rusty, can someone check my f=ma math
We should have gotten ride of Imperial a long time ago.
Fe
RE: Rusty, can someone check my f=ma math
Fe
RE: Rusty, can someone check my f=ma math
Tobalcane
"If you avoid failure, you also avoid success."
"Luck is where preparation meets opportunity"
RE: Rusty, can someone check my f=ma math
Tobalcane
"If you avoid failure, you also avoid success."
"Luck is where preparation meets opportunity"
RE: Rusty, can someone check my f=ma math
The problem with lbf and lbm is that you are applying the same name to two different things. You cannot do this if you are going to do accurate calculations. Somewhere in your set of English equations, g appears. You need it to keep appearing in the same place, or you will become confused, and you will get the wrong answer. In college, I took English and metric units, and we had two sets of equations. I use one set.
I don't do lbf and lbm, or kgm and kgf, because, as far as I am concerned, lb is a unit of force and kg is a unit of mass. Weight is a force, a vector directed towards whatever body is attracting the object through gravity, measured by a spring scale. Mass is a scalar, indicating the amount of matter, measured by a mass balance.
RE: Rusty, can someone check my f=ma math
I agree with you, however, unfortunately in my work environment we speak in pounds (lbf or lbm). It even gets more confusing when we don't know the "weight" of something and then use density (which is in Lbm/in^3) to calculate out the "weight" (which is force) but in actually, from a unit point of view, we calculated the mass which is the same as the weight.
Tobalcane
"If you avoid failure, you also avoid success."
"Luck is where preparation meets opportunity"
RE: Rusty, can someone check my f=ma math
I can't help laughing when everyone (myself included) rehashes this stuff that everyone understands, trying to teach others what they already understand.
Although it's not my personal choice, I can understand why some people cling to the US or imperial units or whatever they're called. One thing I really can't understand is why anyone would ever use lb without the suffix m or f. What's up with that?
Peter M. Potstirrer
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