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

 

[SomptingGuy]

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

 

[FeX32]

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.


Fe

 

[FeX32]

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


Fe

 

[Twoballcane]

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”

 

[Twoballcane]

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”

 

[drawoh]

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

 

[Twoballcane]

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”

 

[electricpete]

Yes, yes, I remember, I had lasagna ;-)

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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(2B)+(2B)' ?