×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Question on value of force in structural calculation
2

Question on value of force in structural calculation

Question on value of force in structural calculation

(OP)
Hi everybody

I have a basic question.

Suppose I need to provide a support for a section of piping which have valve and some instruments on that. The weight of valve and instruments in catalog are given as 75 kg. So do I need to consider it as weight of valve and in calculation use 75 N for force or should I multiply by 9.81 to convert it in weight (W=mg) and then use as force in calculation?

Similarly if the fluid density is given as 1200 kg/cubic meter is it mass density or weight density for the purpose of calculations?

Thanks in advance

RE: Question on value of force in structural calculation

If you really think 75kg gives you a 75N force, what planet are you from?  Really, 75N is NOT the force you get from a 75kg mass. If you don't know what to multiply by, hire someone.

"Keep your units consistent"
If you're designing in metric, keep it in metric.
If your're designing in imperial, keep it in imperial. As many times as they beat it into our brains at school, it makes complete sense to me now.
 

RE: Question on value of force in structural calculation

(OP)
Thanks ChipB for your reply!
The essence of my question is that the value given in vendor catalog under heading of "weight" as 75 Kg is mass or weight?
If I follow unit then it should be mass. If it is mass then definitely I won't get the weight as 75 N.

And if it represents the weight then it should be 75 N (why it is mentioned in kg)unless it doesn't mean Kgf implicitly.
Till now in all text book problems I got either mass or weight explicitly so never faced such problem.
I am just thinking of a situation to solve this problem.
I do not have anything to design.
Would you like to get hired in case I need someone smile

I am very confused on this that's why seeking help from knowledgeable persons.

  

RE: Question on value of force in structural calculation

I typically do all of my design using US stardard units. I would take the 75kg and multiply it by 2.2lb/kg and design for 165 lbs.  

RE: Question on value of force in structural calculation

Well the thing to do is to get the proper value of the weight. Do you think the things would weigh 75 kgf or 7.5 kgf? If you know it is one of the two cases, then follows that if the second you should use 75 kN as an approximation of the weight, and 750 otherwise.

RE: Question on value of force in structural calculation

Look at the material and dimensions of the valve and determine which one makes sense: 75N or 75kg

It is common for us Americans, to use mass as weight.  We know it is not interchangeable, but honestly, I'd take 75kg and multiply by 2.2 to get my weight in lbs, when actually, I got it in lbm.

RE: Question on value of force in structural calculation

Oops.  See, I did it again.  I'd get lbf not lbm

RE: Question on value of force in structural calculation

indimech,

   Watch your terminology and units.  Kilogram is a unit of mass.  Newtons and pounds are units of weight.  If you get confused about this, you will get wrong answers.  Stay away from lbm and kgf.

   w = mg = 75kg × 9.81m/s2 = 734 kg.m/s2

   A kg.m/s2 is a Newton.

   1lb equals 4.45N, not .453kg!

               JHG

RE: Question on value of force in structural calculation

Mass and weight are not interchangeable.  Mass is a material property, weight is a force.  Structures are designed to resist forces.  Multiply the mass (Kg) by acceleration due to gravity (m/s^2) to get the force that needs to be resisted (N)

RE: Question on value of force in structural calculation

i started with a different reply, but ended up with this (which looks wrong to me, but ...)

if the unit weighs 75 kgf = 165 lbf = 734 N

if the unit's mass is 75 kg, then it's weight is 735 N = 165 lbf

ie it doesn't matter !

where's the mistake !?

RE: Question on value of force in structural calculation

rb, you mistake is that's valid only on the earth's surface :)

RE: Question on value of force in structural calculation

Indimech,
Some of the above is correct, some not.  I won't try to straighten it out.

Structural engineers who use the SI system usually simplify mass to force conversions by just using multiples of 10 to get to force.  Thus, for our purposes, 100kg equates to 1kN force.  1000kg is 10kN or 1 tonne.  The fluid with density 1200 kg/m^3 gives a pressure of 12kPa if applied over an area.  This way simplifies things for structural design, and is only a little bit conservative.   

RE: Question on value of force in structural calculation

If the vendor catalog says the weight is 75 kg I would read that as a mass of 75 kg.

Mass and weight are not interchangeable, but many people use the terms as if they are.

RE: Question on value of force in structural calculation

Physics 101

RE: Question on value of force in structural calculation

For years I taught first and second year physics to engineering students. I thought I was overdoing it by trying to emboss the meaning of mass and weight in engineering terms into the brains of the students. Now I can see that you can never do enough to teach engineers about the proper use of units. They could make deadly mistakes if not taught properly.

RE: Question on value of force in structural calculation

indimech,

"weight 75kg" means "mass 75kg".

RE: Question on value of force in structural calculation

Call the manufacturer and ask which it is.

DaveAtkins

RE: Question on value of force in structural calculation

(OP)
@ aayjaber
I feel the problem is not the consistency of units as long as I am working in SI. (I am not much familiar with USCS or FPS or any other for that matter).
My basic confusion is the engineering aspect of mass and weight. I am pretty clear about difference between mass and weight in terms of physics.

To further simplify my question generally I measure my "weight" on weighing scale and it reads 50 "Kg". So it this 50 my weight or my mass? And if I want to design a chair for me to sit should I use the load as 50 (considering the measurement as weight)or 50 x 9.81 (considering the measurement as mass)

If instead of weighing myself on weighing scale I weigh on a balance scale would i get different value?

RE: Question on value of force in structural calculation

When the general public say 'weight' they mean 'mass'.  Your scales are telling you your mass.  Kilogram (kg) is a unit of mass. Newton (N) is a unit of force.  If you use a balance scale and you put 50kg on one side, what do you put in the other?  Does it come with a selection of 50N weights?  Or would you use weights given in kg?

The unit will identifies itself.  If it said 50Hz what would that mean? Or 50 Volts?

If your supplier says weight 75kg, to me that means mass of 75kg equal to a force (on earth) of 734N.  Which is your static load.  If they said weight 75N then that is your static load.  If they didnt give you any units, and just said 75 then your stuffed.

RE: Question on value of force in structural calculation

736N, my mistake

RE: Question on value of force in structural calculation

indimech,

   You sound vague and confused.  "Weighing scale" does not mean anything to me.  

   A spring scale measures the force due to gravity.  This is weight, in units of Newtons and pounds.

   A balance compares your mass with that of some standard masses.  The units are kilograms and slugs.

   When you do your calculations, try doing a unit balance.  If you are doing this wrong, your units will not make sense, and you will know you have to fix something.  

               JHG

RE: Question on value of force in structural calculation

When you stand on a weighing scale and measure your weight, its unit is not kg-m but kg-f. It is the measure of the earth's pull towards the core. Your true mass = weight in kg / 9.81. Your mass is always a constant, but your weight will change depending on where you are (earth/moon etc)

RE: Question on value of force in structural calculation

(OP)
After getting so many responses I think its good to conclude that if an object is weighed on spring scale, the weighing scale reading will show its weight and to include that weight (force) in calculation as static load I need not to convert that weight into force by multiplying with acceleration due to gravity.

On the other hand if the object is weighed on balance the value of the mass against which it has been balanced will give me the mass of that object and to include that in calculation as static load I need to convert that mass into weight (force).

And obviously I will have to contact the vendor to know if the shown value on the catalog is measured by spring scale or balance. I would consider myself lucky if he knows that.

Otherwise better to consider the value on catalog as mass convert to weight and do the calculation. Some conservatism may come in the results but in view of all uncertainties it would be acceptable.

Any other thoughts are welcome.
Thanks all for the discussion.

RE: Question on value of force in structural calculation

uh ? ... if you weigh something (spring scale, balance beam) you get it's weight.  

i think i was correct in pointing out that a weight of 75kg is equivalent to a mass of 75kg (pretty obvious when you express it that way !), the force is 736N.

RE: Question on value of force in structural calculation

Amazing that this issue has gone on so long without resolution.

If the "weight" is in kilograms, it is mass, and should be multiplied by "g" to get to force.

If the "weight" is in pounds, it is a force, and no multiplication needs to be done.

It doesn't matter what type of scale was used.  If you put a 5 pound bag of sugar on a balance scale, the corresponding object that balances it will have a weight of 5 pounds.  They also have equal masses, but nobody wants to think about slugs.

RE: Question on value of force in structural calculation

I find this discussion quite astonishing, but I'm glad that nutte has brought up 'pounds' to confuse the issue.

Going back to my schooldays I seem to remember that a pound is a mass but a poundal is the force, but nobody says poundal and confusingly calls the force pound, the same as the mass. Numerically though, one is different to the other. I'm just glad that here in the UK we switched to metric where mass and weight have conveniently different names and nobody gets confused.... or do they?

More slugs please.

corus

RE: Question on value of force in structural calculation

rb1957,
You stated:

Quote:

i think i was correct in pointing out that a weight of 75kg is equivalent to a mass of 75kg (pretty obvious when you express it that way !), the force is 736N.

That statement may be true at sea level, but not on a mountain top, not in a valley and certainly not on the moon.

BA

RE: Question on value of force in structural calculation

BA,

you're quite right, but i'm pretty sure that 99.99% of engineers would use 32.174ft/sec2 (or 9.807m/sec2) for "g".

and if the application was for extraterrestal then the OP would state so (probably not at first, but usually in a later post "oh, btw, this is on the moon")

RE: Question on value of force in structural calculation

(OP)
nutte,
I talked about spring weigh and balance because when I go to purchase any commodity it is measured in kg. The ref weight they keep on one side of balance say 1 kg and the commodity on other side say sugar.
I wanted to know whether that 1 kg ref weight represents mass and gives mass value or it is weight. and I will not get same "numerical value" if i compare an object with a "mass" and then weigh on spring scale unless spring scale is calibrated to show mass reading.

It is correct for lbm and lbf on earth since coincidentally both are numerically equal on earth.

 

RE: Question on value of force in structural calculation

A kilogram is a unit of mass.  Period.  End of discussion.

RE: Question on value of force in structural calculation

(OP)
I guess I got the concept.

When weight is given in pounds there is no need to think about converting to force because pound (mass) and pound (force) both are numerically equal on earth. Hence same numerical value shall be used as static load.

But when weight is given as Kg it implies Kgf and we need to multiply by g to convert to force to use as static load.

The confusion arouse due to dropping of 'f' from Kg due to convention. Weight 27 Kg means 27 Kgf.

The following vendor site gives weight in pounds (without mentioning lbf or lbm)and Kg.

http://www.fisherregulators.com/products/literature/sr8/

If I compare both values then it comes that the value in Kg is weight in kgf and value in pounds is also weight.

This lets me deduce that both values are in force units lb in lbf and kg in kgf but f is omitted for simplicity.
thats why
27 kgf becomes 27x9.81=264.87N
and 60 lbf equals 60x4.45=267N

 

RE: Question on value of force in structural calculation

indimech,

   Your grocer is not doing acceleration calculations, so it is harmless for them to confuse units.

   When I was in college, we learned English units and metric.  We learned one set of equations for acceleration equations using force units, and another set of equations using mass units.  You can analyze force using lbm and kgf with gay abandon if you select the correct set of equations every time.

   There is a simpler way, less likely to result in mistakes.

   m = w/g

   f = ma = (w/g)a

   m is in units of kg or slugs.  In English calculations, if you insert (w/g) for mass, you never have to see a slug.

   w is in units of N or lb.

               JHG

RE: Question on value of force in structural calculation

I'm with corus on this.  I have never heard of lb as a unit of force so I guess this must be an American convention.  To me 1kg = 2.2lbs, both units of mass.

RE: Question on value of force in structural calculation

i'm with corus, this had gone on way too long, and not gotten far, based on the last couple of posts.

"when weight is given as Kg it implies Kgf and we need to multiply by g to convert to force to use as static load" ... no, no, no.  the "f" in kgf stand for force; when weight is given in kg (or kgf) it is a force and we Don't have to multiply it be anything to get force.


ussuri, "To me 1kg = 2.2lbs, both units of mass" ... absolutely correct except for the last word, which makes it absolutely wrong.  a 1kg bag of sugar is equivalent to a 2.2lb bag of sugar ... it's weight not mass.  i'm guessing you're in europe or some other place that hasn't seen the imperial system ... pounds have been used for weight for an awful long time, longer than N (or kg).  

the problem is the metric system introduced the kg as a force (i guess the prols couldn't handle a Newton) and messed up a very nice system.  however, the saving grace is that the weight of a 1kg mass is 1kgf (so long as your g = 9.807m/sec2).  the only time to be careful is when you need the mass of something, like for momentum calcs, or mass flow rate.

RE: Question on value of force in structural calculation

(OP)
Sorry I made wrong statement.

"when weight is given as Kg it implies Kgf and we need to multiply by g to convert to force to use as static load"

It should be:

"when weight is given as Kg it implies Kgf and we need to multiply by g to convert in 'Newton' to use as static load"

RE: Question on value of force in structural calculation

geez, no, no, no ... kgf is a force. the correct statement is ...  "when weight is given as Kg it implies Kgf and we use as static load"

sometimes people are confused (lazy?) and say "weight" when they mean "mass" (and vis-a-versa).

RE: Question on value of force in structural calculation

I'm flabbergasted.


Quote (Ussuri):

I have never heard of lb as a unit of force so I guess this must be an American convention.  To me 1kg = 2.2lbs, both units of mass.
That is wrong wrong wrong.  Pound is a unit of force.  The corresponding mass has the unit "slug."


Quote (rb1957):

a 1kg bag of sugar is equivalent to a 2.2lb bag of sugar ... it's weight not mass.
This is also wrong.  Kilograms are a unit of mass.  Pounds are a unit of force (weight is a force).  A 1kg bag of sugar has the same mass as a 1lb bag of sugar.  They also have the same weight.  But that doesn't mean the two numbers with different units are the same.  The weight in force units of a 1kg bag of sugar is 9.81 N (Newtons).  9.81N and 2.2lb are the same, and both are units of force.

Back to the original question:
Kilograms measure mass.
Pounds measure force.
To get the force induced by a 75kg mass (on earth, at sea level, when you hold your nose just right), you multiply it by g.  The force is 75*9.81=736N.

This should have been resolved with the second post.

RE: Question on value of force in structural calculation

(OP)
Since I am more comfortable with working in Newton as force than Kgf as force that's why I wrote to convert it into Newton.

But Kg implies force when given as weight that's the crux of discussion.

RE: Question on value of force in structural calculation

Off topic.

rb1957, pound is a unit of both mass and force. I disagree that pound as a mass is 'absolutely wrong'.

Thankfully, being British, and not old enough to remember imperial I do not have the problem of sorting out pound-force and pound-mass.
 

RE: Question on value of force in structural calculation

Quote (indimech):

But Kg implies force when given as weight that's the crux of discussion.
It may imply force, but it's giving mass.  Kilograms are always mass.

RE: Question on value of force in structural calculation

A kilogram is a unit of mass.  A 1 Kg bag of sugar has a mass of 1 Kg on the earth, on a spaceship or on the moon.  But its weight varies according to the acceleration of gravity in its particular locale.  I am amazed that we are arguing about this.

BA

RE: Question on value of force in structural calculation

nutte, BA

when was the last time you were in a store ? buying a kg of sugar ??

 

RE: Question on value of force in structural calculation

(OP)
Guys,
I started this discussion to clear my doubt why a vendor gives a mass unit in the heading of weight and whether he means that given value represents the mass or weight?

Please see the website for my doubt:

http://www.fisherregulators.com/products/literature/sr8/

Now after reading the point of views of so many persons and probably from different geographies of earth where different system of units are followed I got more insight in this topic which I want to outline:

1. Before this discussion I used to think pound is unit of mass.
2. In USCS (United States Customary System) pound is unit of "Force" and Slug is unit of "Mass". Where as in FPS unit system pound is unit of "Mass"
So depending upon system of units it represents both.
But on earth co-incidentally pound force are pound mass are equal.
2. We get groceries or commodities on weight basis so if i purchase 1 pound sugar it means that sugar "weighs" 1 pound (lbf) similary where things are sold in Kg it means the weight of sugar I purchased will be 1 Kgf and for convenience they omit 'f' and if I convert this in newton it will become 9.807 N

1 lbf sugar = 4.45N
1 lbm sugar = .453 Kg of sugar (mass basis)
weight of .453 Kg = .453x9.807=4.4425 N

On earth due to the value of proportionality constant of Newtons second law equation F= Kma, lbf and lbm are equal.

3 The direct equivalent of Kg (mass) is NOT Pound (mass). It is Slug.

4. If you are using mass in Kg or Slug then you can directly use the equation F=ma (K is equal to one in these cases) but if you are using mass in pound you have to use F=kma.

5. If a vendor writes weight as 27 Kg he means 27 Kgf to get the value in newton it should be multiplied by 9.807 for those who work in N. For those who work in Kgf as force no need to multiply by 9.807.

If weight is given in pound it is basically pound force.
If anybody uses USCS there is no confusion since he is incorporating force in his calculation.

Those who work in FPS taking pound as mass if they convert it in force say pound force they will get the same value.
For example 60 pound mass on earth will be 60 pound force.

    

RE: Question on value of force in structural calculation

rb1957,

I don't buy sugar.  It's not on my diet.

BA

RE: Question on value of force in structural calculation

1 lbm * 32.2 ft/s^2 * 1/32.2 lbf*s^2/lbm ft = 1 lbf

1kg * 9.8 m/s^2 * 1 N*s^2/kg*m = 9.8 N

(Mass * g)/gc = Force

Going against the grain, I'm going to have to say I like the FPS system better...  1 = 1!  (as long as you are religious about indicating lbm vs lbf it works very nice).  

On the subject of vendor data-sheets claiming "weight" in kg.  They do that.  Sometimes I think the datasheets are produced by the sales department.  I would hope you can figure out by the factor of 10 what is meant.   

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources