Rusty, can someone check my f=ma math

[shorton2]

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

 

[desertfox]

Hi shorton2

Yes looks right to me

desertfox

 

[zeusfaber]

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

A.

 

[dvd]

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.

 

[shorton2]

<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

 

[IRstuff]

Your calculation is correct, as far as it goes. The high CG may cause some rocking of the load, so your motor might need to deal with that as well.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[shorton2]

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!

 

[Szasza]

my advice is: please use SI!!!!

 

[rb1957]

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)

 

[SomptingGuy]

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

- Steve

 

[imcjoek]

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.

 

[rb1957]

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

 

[drawoh]

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

 

[rb1957]

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.

 

[electricpete]

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)' ?

 

[TheTick]

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

 

[Twoballcane]

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”

 

[kingnero]

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.

 

[shorton2]

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

 

[Twoballcane]

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”