Why is mass calculation actually a weight calculation in iProperties Phys. tab?

[wisco12]

It has the correct units of lbmass, but the number is actually measuring the weight in lb force.

For instance, it says my assembly has a mass of 200 lbmass. But I know that in reality it actually weighs 200 lb force. Let me point out that lbmass and lbforce are not the same thing. If I step on a scale, it measures my weight and not my mass.

lbforce = (acceleration of gravity)*(lbmass)

So you either should change it to read Weight instead of Mass (and also change units) or actually calculate the mass of the objection (mass = density*volume).

I don't know how this software could get something so fundamental like this wrong. However, please correct me if I'm the one with the incorrect understanding.

[jalger]

hmmm,

I'm not seeing what your saying:

if i make a 5cmx5cmx5cm block of Gold (as an example).

I get 19.3 g/cm^3 as density and 7.628in^3 as my volume.

Since we need to convert one or the other to do our math we will inheritly have rounding errors... but not really the point.

Convert 7.628 in^3 to Cm^3 (cause why would you want to double convert on the other side?)

We Get 125.000524 cm^3

So,

19.3g/cm^3 X 125.000524cm^3

=2412.51 g (or 2.41251Kg)

Convert to pounds

5.3186 lbs (mass) inventor tells me my mass is 5.319...

So Again I'm not sure where the issue is?

In this case your saying it should say : 23 KN (KgM/s^2)

lbsforce = G (Gravity or 9.81m/s^2) x lbsmass (5.319lbs or 2.412kg)

= 9.81 x 2.412

= 23 KN (KgM/s^2)

If you leave the part on "Default" It doesn't giving any weight since its not a proper density.

I would say it looks right to me.

Mass = Density * Volume

I'm I missing something here?

Regards,

James

[graemev]

You are correct.  Imperial units should be in either poundals or slugs. 

Write your congressional representatives about catching up with the rest of us.

[wisco12]

Okay, so my problem stems from the definiton of the lbmass.

This youtube video explains that lbforce = lbmass.

https://www.youtube.com/watch?v=4ePaKh9QyC8

However, this Wikipedia is very confusing.

https://www.youtube.com/watch?v=4ePaKh9QyC8

Near the top, it defines 1 lbf = 32.2 ft*lbm/s^2

But if you look at the system of units table, the EE column suggests that lbf = lbm, at least when g = g_c (which is true on Earth).

I think that Wikipedia article needs to be revised because I think I was previously mistaken. I now believe that 1 lbf = 1 lbm. Which I admit is just really strange for me. I was taught mass and force being two distinctive entitites, but the EE system of units tries to make them the same.

In the standard English unit system: force is in pounds, mass is in slugs, and gravity is in ft/s^2. And they are related by f = ma.

Whatever, I think the lbmass is just a stupid unit. SI and standard English are much clearer.

I'd like it much more if Inventor either used mass in slugs (English) or in kg (SI).

Now looking at your work:

You state that 2.41251 kg = 5.3186 lb. This is true. However, in SI or standard English the 5.3186 is NOT mass; it is force. In SI, it's Newtons (N). In standard English it's pounds (lb). You were converting 2.41251 kg (a mass quantity) to 5.3186 lb (a force quantity, at least in SI and English).

Now the weird part is in EE it is both force and mass, which is the system I guess Inventor uses.

Sorry this got lengthy, basically all my troubles stem from not knowing what lbmass was. 1 slug = 32.2 lbmass.

Thanks

[jalger]

No it was mass to mass...

2.20462lbs (Mass) = 1 KG (Mass)

[wisco12]

Well in my opinion you need to clarify which type of pound you're dealing with. From what I know, the slug is the preferred measurement of mass in the United States (or kg if SI is preferred). Pound mass will have equal scalar quantity as pound force, but one is a mass quantity and the other is a force quantity. And typically, lb alone will be interpreted as the force version.

If you're going to use the F=ma equation, this is an important distinction. You can't take 1 lbmass*32.2 ft/s^2 to get lbforce. You need to convert lbmass to slugs in order to use that equation (it's analogous to gram vs kilogram).

But yes, I was previously wrong about lbmass. Inventor gave me the correct mass in lbmass units, I just didn't know you can't take lbmass*gravity to get lbforce.

I have no idea why this was so difficult for me to grasp in the first place. I undertand it now.

[jalger]

Great!

How many Slugs do you weight?  or rather whats your mass in Slugs?

Unless you deal with Engineers, and Technical people all day I'm betting if you ask this question of most people you will get Pounds (LBS) or Kg as an answer.

Most people deal with LBS and KG for shipping, receiving, or just to tell people how much something weighs.

So if slugs are perfered, were are they used?  Test Labs?  

Even in industry (Automotive, Medical, Automation) I have not encountered someone wanting the Weight in Slugs.

I sure haven't seen my mail weighed that way. (I live in Canada so I see LBS and KG on my packages.)

When I get packages from the states or overseas I have never seen Slugs on my package for weight. 

You stated:

"Well in my opinion you need to clarify which type of pound you're dealing with"

Well if i put lbf its pound-force and LBS is generally assumed to be mass. or I could put lbm (but then people mistake it for other things).

Look I'm not trying to spark an argument over which unit of measure is better.

You asked if it was wrong... and no its not.

F = ma is right for lbf .

1lbf = 1lbm * Gravity (32.2 ft/s^2)

just as

1N = 1g *9.81 M/s^2

Regards,

James

[jalger]

Great!

How many Slugs do you weight?  or rather whats your mass in Slugs?

Unless you deal with Engineers, and Technical people all day I'm betting if you ask this question of most people you will get Pounds (LBS) or Kg as an answer.

Most people deal with LBS and KG for shipping, receiving, or just to tell people how much something weighs.

So if slugs are perfered, were are they used?  Test Labs?  

Even in industry (Automotive, Medical, Automation) I have not encountered someone wanting the Weight in Slugs.

I sure haven't seen my mail weighed that way. (I live in Canada so I see LBS and KG on my packages.)

When I get packages from the states or overseas I have never seen Slugs on my package for weight. 

You stated:

"Well in my opinion you need to clarify which type of pound you're dealing with"

Well if i put lbf its pound-force and LBS is generally assumed to be mass. or I could put lbm.

Look I'm not trying to spark an argument over which unit of measure is better.

You asked if it was wrong... and no its not.

F = ma is right for lbf .

1lbf = 1lbm * Gravity (32.2 ft/s^2)

just as

1N = 1g *9.81 M/s^2

Regards,

James

[wisco12]

Don't worry, I'm not looking for an argument either!

You're absolutely right about pounds mostly be considered a mass. However, when you ask how much I weigh, you are looking for how much force I exert which is also in pounds. When I was talking about slugs I was only thinking about engineering and technical fields, so you're right about pounds being most common for anything else. I think my confusion is from my high school teachers trying to drill into us the difference between mass and weight (physics teach said pounds was a force, not mass. But now I know about lbmass which is a mass).

I was with you until the very end...I do have to disagree with you on your last comments.

1 lbf DOES NOT equal 1 lbm*32.2 ft/s^2. That is wrong because we know on Earth, 1 lbf = 1 lbm which is why you can use the generic pound for both.

Rather, 1 lbf = 1 lbm*( 1 slug/32.2 lbm)*32.2 ft/s^2. 

                    = 1 slug*ft/s^2

                    = 1 lbf

Likewise, 1N DOES NOT equal 1g * 9.81m/s^2. (Note that this is wrong on both scalar and unit basis)

To convert a gram to Newton, you do this:   1 g * 1 kg / 1000 g * 9.81 m/s^2  = 0.1111 N

Best wishes

[jalger]

LOL Yeah  two things wrong with my statement, 1. I missed the "Kilo". 2. 1 N doesn't equal 9.8n...LOL

Should say Force in Newtons  =1Kg * 9.81M/s^2  ((Kg-M)/s^2)

1n= 1(Kg-M)/s^2

Not sure about the slug thing your end units wouldn't be correct.

lbf  is ((lbs-ft) /s^2) most of the time its writen ft-lbs (foot pound). It Should be writen as (lbm-ft / s^2) but most people are just plain lazy...

your teacher would want to see you put the proper lbm not lbs.

If you started with a value of 5 lbs gravity exerts a force of 32.17 ft/s^2 on the object.

even without accelerateing the object has a force on it in this case it would be 160.85 ft-lbs/S^2 or 160 lbf or  5 slugs

Another example:

1lbf=1lbm *32.17 ft/s^2

=32.17 (ft-lbm)/s^2

1 slug = 32.17 lbm

and 1lbm = 1 / 32.17 slug

1lbf =1bm *G

1lbf = 1/32.17 *32.17

1lbf = 1 slug-ft / s^2 

So, what I'm saying is your not really wrong, I just don't get why your doing the unit conversion if you don't have to. (But I'm guessing you were taught to give the number as Slugs; not in Foot-pounds)

Something you may have not learned yet is that you burn energy just by existing. (i.e. just siting causes you to burn engery to counter act the force of Gravity (and for chemical reasons too!) If you were to travel into space your body consumes less energy since it doesn't have to fight gravity. (A side effect is your muscles grow weaker from not being in use).

If you could land on a planet with twice the gravity of earth your mucles would become twice as strong (assuming your survive long enough for your muscles to adapt), Once you leave that other planet your musles and body would adapt again to what ever the environment it is in (so no super strength for you sorry).

Another interesting thing is that gravity can vary (i.e. its actually not the same everwhere on the planet) it can increase or decrease based on several factors.

but again not really the point. magnetic fields, dense material (like a lead mine) would create abnormal gravity for the local area. (it doesn't vary much, we are talking about +/- 0.7%)  Thats why we average out the values. 

Glad to see that you are using Inventor to setup the problems and check your solutions,

I hope that you continue to use it.

Regards,

James

[wisco12]

Thanks for the reply!

I think you have mistaken the slug for a unit of force for a moment here:

>"even without accelerateing the object has a force on it in this case it would be 160.85 ft-lbs/S^2 or 160 lbf or 5 slugs". It should read 5 slug-ft/s^2.

>"Not sure about the slug thing your end units wouldn't be correct".

I looked over my units and I think they were correct. 1 lbf = 1 slug-ft/s^2

The units you provided in your latest message are correct too!

And yeah the reason I do the unit conversion is because we were taught to use slug for imperial and kg for SI. Never had to use lbm.

I get the convenience behind using lbmass (it's the same magnitude as lbforce so you don't have to compute anything). I've never expressed a force in the ft*lbm/s^2 units before. It doesnt really make intuitive sense when you can just express a force in lbf or N.

From now on, if someone tells me an object has a mass of 5 lbm, I will know without doing any calculation that it has a weight of 5 lbf.

[dan_inv09]

So Inventor is putting things I draw on a scale?

When Inventor calculates the volume and the multiplies it by the density it is not getting the mass of the objection [sic] (mass=density*volume)?

I'm sorry, I just can't get my head around what you are asking.

[wisco12]

I didn't know lbmass (a mass unit) has the same magnitude as the lbforce (a weight unit) on Earth.

Read the other comments for more information. It's all sorted out. My mistake, not Inventor's.

[dan_inv09]

The understanding I always had was:

1 lb [force i.e. weight] = 1 lbmass x 1 earth-gravity

Nothing against anybody else here but reading the other comments had only confused me more. (Not to name names, but someone took the time to caution against rounding errors in the middle of specifying that 5 cm x 5 cm x 5 cm = 125.000524 cm^3 )

[jalger]

Err...no

5lbf (FORCE) is not 5lbm (MASS)

hmmm new approach.  (FORCE DOESN"T EQUAL MASS)

F=m*a

F=M*G  where Acceleration (a) = Gravity (G)

 F

----  =  m 

 G

            1 lbm * ft

1lbf =  -------------   (This is F)

                s^2

G = 32.17 ft/s^2

So

        1lbm*ft

        ------------

           s^2

m=   ------------

         32.17Ft

         ----------

           S^2

so math rules - Change the division to multiplication (So flip the Units to cancel out and balance our units)

       1lbm *ft             S^2

m=  -----------  X  --------------     (ft and S^2 are canceled out leaving lbm)

           S^2               32.17 ft

            1

m =  -------   lbm

         32.17

I think I understand where your confusion is now:

Slugs, lbm and KG are MASS

lbf, N, and Kp are FORCES 

lbf is derived from Mass and Acceleration  (Techincally it's an Unbalanced force).

Unit conversion always throws a wrench in things because people forget to cancel out the units.

For Dan-

And Yeah I Should have used 125.0cm^3  lol, I just did a direct conversion from the value in inventor. (it probably had more digits I just got lazy)

And in one of my posts I forgot the K for KiloGrams. I'm only human and make mistakes.

(I Probably have a few other errors in there)

Bottom Line is 5lbm is not 5 lbf.

Regards,

James

[wisco12]

I know that man! Geez you are making this way harder than it has to be.

I know lbf and lbm aren't exactly the same, lol.

All I said is that they have the same magnitude or scalar quantity. 5 = 5.

That's why it's confusing. Usually your weight won't have same scalar quantity as your mass, except for this pound mass unit system which relies on this convenience.

You seriously thought I didn't know the difference between mass and force?

Slugs is nice because you can multiply it by gravity to get force in pounds straight up. You can't do that with lbmass because you have to convert ft-lbm/s^2 to lbf.

Pound mass is nice because you know it will have same scalar quantity as your force. Got 5 lbm? Then you got 5 lbf. (on Earth). But can't straight up plug it into F=ma equation without doing a conversion.

Hopefully you don't have any issues with any of the above. Let me know if I made a mistake.

[dan_inv09]

So you are trying to say that one pound of force is a thirtysecond of the amount of force a pound of mass will experience in earth's gravity?

Or, if I had a physical piece of something that Inventor says is 32.17 lbmass, and put it on a scale it would say 1?

[wisco12]

Nope I'm saying one pound of force corresponds to one pound of mass. If you have one pound of mass (1 lbm), it will exert one pound of force (1 lbf) in Earth's gravitational field.

[jalger]

And that's Why I Avoid using lbs and lbf.

Give me N and KG anytime Why? Cause then there is no way to confuse one value for another.

M =5lbs = 2.26796 KG

A  = 9.81m/s

F= M*A

F= 2.26796 *9.81

F=22.25N (Convert back and get 5.00 lbf)

Dan / wisco You Are 100% Right I screwed up that calculation.

Regards,

James

[wisco12]

Yup I agree. It would be a lot less confusing if they both weren't called pounds. Also, I just wanna say the reason I think they can get away with calling them both pounds is because they have same scalar quantities.

Thanks for your time in resolving my issue!