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Rusty, can someone check my f=ma math

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



 

RE: Rusty, can someone check my f=ma math

Hi shorton2

Yes looks right to me

desertfox

RE: Rusty, can someone check my f=ma math

I'm dying to know why you want to accelerate a bathroom scale sideways!

A.  

RE: Rusty, can someone check my f=ma math

I'm kind of thinking that the weight of some object was measured on a bathroom scale, and that it is not a 20 lb bathroom scale.  20 lb for a bathroom scale seems kind of heavy, but I guess that depends on the local accelerations.

RE: Rusty, can someone check my f=ma math

(OP)
<snicker>

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

(OP)
Understood.  It's all working pretty well.  I've got a reasonable accelleration ramp so its not rocking much.   This is a worst case calculation.  I'm just running the bearing life calculations and verifying limits have a lot of safety factor.

Thanks guys!

 

RE: Rusty, can someone check my f=ma math

my advice is: please use SI!!!!:)

RE: Rusty, can someone check my f=ma math

i'm not so sure ... the 20 lbs is 20 lbf (weight) not 20 lbm ... i think you need to divide by g (32.174)

RE: Rusty, can someone check my f=ma math

I'm with Szasza, do all your woring using SI.  Convert back to your favourite agricultural units afterwards.

- Steve
 

RE: Rusty, can someone check my f=ma math

20lbf = 20lbm at traditional 32.2ft/s^2 gravity.  You divide by g, but have to multiply by gc for the units to work.  This conversion is true of SI as well.

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

a different way to look at it is to say your acceleration is roughly 1/3g, so the force is 20/3 = 6 lbf

RE: Rusty, can someone check my f=ma math

Quote (Szasza):


my advice is: please use SI!!!!:)  

   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.  
 

               JHG

RE: Rusty, can someone check my f=ma math

crap !  i should learn to read ... he's divided by g and got the right answer (sigh), 2 lbs.

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

FWIW - I'm born and raised with lbm, lbf etc, but I have a hard time agreeing with the suggestion that there is less potential for confusion in the lbm, lbf etc unit system than with SI.

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.

=====================================
(2B)+(2B)'  ?

RE: Rusty, can someone check my f=ma math

I hate English units for this sort of thing (arcane mass/weight relationship).  I usually convert to metric, calculate, then convert back.

RE: Rusty, can someone check my f=ma math

OK here is the low down in English.  F=ma...Lbf=Lbm(ft/s^2).  Now everything you weigh on a bathroom scale is mass (ie lbm, Kg).  In english there is a converter called a conversion constant "gc" which is 32 (lbm-ft)/(lbf-s^2).  Thus for English F=(m/gc)a.  So the OP unwittingly did the right thing.

Tobalcane
"If you avoid failure, you also avoid success."
"Luck is where preparation meets opportunity"  

RE: Rusty, can someone check my f=ma math

That's why it's great to live in europe...

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

(OP)
Who says it was unwittingly :)

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

Hmmmm :) how did you know that you were going from pound mass to pound force?  The units you used did not indicate this.  Now from the questions and conversion supplied is your weight in pound mass or pound force?  Be careful, it's a trick question.  Now in SI you can have two different numbers, one in newton and one in Kg.  But in english it's.....

Tobalcane
"If you avoid failure, you also avoid success."
"Luck is where preparation meets opportunity"  

RE: Rusty, can someone check my f=ma math

(OP)
OK, from memory then. Using the units that after cancellation left me with what I wanted.  But as I wasn't sure by any stretch, and was compelled to ask, so certaitnly a little unwit <big grin>.



 

RE: Rusty, can someone check my f=ma math

shorton2,

   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).

               JHG

RE: Rusty, can someone check my f=ma math

(OP)
Understood.  Thanks.  I had it right all along.  Just wanted to be sure.

 

RE: Rusty, can someone check my f=ma math

Both English and SI users tend to describe "weight" in mass units, i.e., pounds and kilograms.  I've seen many documents describing a system weighing "x" kilograms.  The meaning is generally clear in both cases.  Someone stating a weight as "x" pounds is referring to the mass equivalent of 2.2 kilograms.  Rarely does anyone talk about weight in slugs or newtons; it's generally not done because most scales are calibrated to display weight in mass units.   

TTFN

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RE: Rusty, can someone check my f=ma math

Quote (IRstuff):

Both English and SI users tend to describe "weight" in mass units...

   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.  

               JHG

RE: Rusty, can someone check my f=ma math

I did not mean to offend anyone. Just I can not to calculate in English parameters, because I learn the SI, and it's hard to convert for me. It' s hard because 1"=25,4 mm or 1kg=2.204623 pounds..it' s crazy for me. sorry!

RE: Rusty, can someone check my f=ma math

What drives me nuts is the use of "g-cm" for torque.

RE: Rusty, can someone check my f=ma math

Quote (TheTick):


What drives me nuts is the use of "g-cm" for torque.

   I do not approve of that either, then again, how about converting N.m torque into Joules?  

   thread1183-295198: Bending moment calculation

 

               JHG

RE: Rusty, can someone check my f=ma math

How about mass moment of inertia in english...Lbm-in^2...what will your Lbm be in?  What you weigh on your bathroom scale or calculated?

Tobalcane
"If you avoid failure, you also avoid success."
"Luck is where preparation meets opportunity"  

RE: Rusty, can someone check my f=ma math

it should be slugs (not lbm) ...
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

Shirley as we can see from the above, Imperial is the way to go pipe

peace
Fe

RE: Rusty, can someone check my f=ma math

Oh and I agree with rb smile

peace
Fe

RE: Rusty, can someone check my f=ma math

Twoballcane,

   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.

               JHG

RE: Rusty, can someone check my f=ma math

so long as you remember that g is 386 in/sec^2
and i'd add the "f" to "lb"

RE: Rusty, can someone check my f=ma math

The moment I acquired an intuitive grasp of the difference between mass and weight, I also developed a visceral distaste for English units.

RE: Rusty, can someone check my f=ma math

but weight is mass just like kg is mass right?  So weight is Lbm  so be careful with w/g.  That is why I use F=(m/gc)a for english.  Many say to covert lb to kg it's the same thing, mass.

Tobalcane
"If you avoid failure, you also avoid success."
"Luck is where preparation meets opportunity"  

RE: Rusty, can someone check my f=ma math

weight is not mass ...

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

So what do you get when you convert kg (which is mass) to lb or vice versa?  Did the kg change to something else?  In my 15 year career, I have never had to use slugs, and never had any requirements (military and comercial) given in slugs.  I've done countless mass moment inertia calcs using lbm (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

Quote:

I've done countless mass moment inertia calcs using lbm (weight).

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

you can, of course, use lbm; it's just that it's not the fundamental unit, slugs are, and you need to be careful with g (which you are using lbf/g which gives you the funny unit of mass moment.  the "correct" imperial unit is (IMHO) slug.ft^2; using lbm you need to include "g" as in I*w^2 is (slug*ft^2)/sec^2 = lbf*ft ('cause 1lbf = 1slug*1ft/sce^2) or (lbm*ft^2)/sec^2/32.17 = lbf*ft ('cause 1lbm*32.17ft/sec^2 = 1lbf)

"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

How easily do the old (oddly named) British units work with electrical units though?  SI seems to be seamless, something that grabbed me by the whatsits when I first realised this back in high school.

- Steve
 

RE: Rusty, can someone check my f=ma math

From the electrical stuff I have done (mainly induction and EM stuff) they work out as seamless as the mechanical units.
We should have gotten ride of Imperial a long time ago.  

peace
Fe

RE: Rusty, can someone check my f=ma math

But then, this thread would have been finished at the 3rd post pipe

peace
Fe

RE: Rusty, can someone check my f=ma math

ding..ding..ding..imcjoek has won the prize from my first question "is your weight in pound mass or pound force? ".  Lbf is the same as Lbm in our gravitational field (g). 1pound weight = 1lbf =1lbm because of F=(m/gc)a or in our gravitational field F=(m/gc)g which is F=m. That is why you can use the weight as lbm in mass moment of inertia calcs.

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 have to concede though that I would rather do everything in SI than English.  It's strange that in college we did everything in SI and then come out to the real world (well here in the USA) that most engineers worked in English.  It drove me up the wall the first five years.  I too use to convert from English to SI then back again, but there are too many steps that you can slip that decimal point.  The science community has embraced SI.  Why can't the engineering community do the same?

Tobalcane
"If you avoid failure, you also avoid success."
"Luck is where preparation meets opportunity"  

RE: Rusty, can someone check my f=ma math

Twoballcane,

   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.   

               JHG

RE: Rusty, can someone check my f=ma math

drawoh,

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

Yes, yes, I remember, I had lasagna winky smile

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

=====================================
(2B)+(2B)'  ?

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