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

 

[shorton2]

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>.

 

[drawoh]

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

 

[shorton2]

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

 

[IRstuff]

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

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[drawoh]

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

 

[Szasza]

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!

 

[TheTick]

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

 

[drawoh]

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

JHG

 

[Twoballcane]

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”

 

[rb1957]

it should be slugs (not lbm) ...
1 lbf = 1 slug*1 ft/sec^2 or
1 lbf = 1 lbm*32.17 ft/sec^2

 

[FeX32]

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


Fe

 

[FeX32]

Oh and I agree with rb


Fe

 

[drawoh]

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/sec[sup]2[/sup]) [&times;] in[sup]2[/sup] = lb.in.sec[sup]2[/sup].

This looks a little weird, but if you convert this unit to MKS, you get...

N.m.s[sup]2[/sup] = kg.m/s[sup]2[/sup] [&times;] m.s[sup]2[/sup] = kg.m[sup]2[/sup].

JHG

 

[rb1957]

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

 

[TheTick]

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

 

[Twoballcane]

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”

 

[rb1957]

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

 

[Twoballcane]

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”

 

[imcjoek]

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.

 

[rb1957]

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).