Question on value of force in structural calculation 2

[indimech]

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

 

[ChipB]

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.

 

[indimech]

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

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

 

[DWHA]

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.

 

[ishvaaag]

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.

 

[ChipB]

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.

 

[ChipB]

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

 

[drawoh]

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/s[sup]2[/sup] = 734 kg.m/s[sup]2[/sup]

A kg.m/s[sup]2[/sup] is a Newton.

1lb equals 4.45N, not .453kg!

JHG

 

[steellion]

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)

 

[rb1957]

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 !?

 

[steellion]

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

 

[hokie66]

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.

 

[apsix]

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.

 

[aayjaber]

Physics 101

 

[aayjaber]

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.

 

[Tomfh]

indimech,

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

 

[DaveAtkins]

Call the manufacturer and ask which it is.

DaveAtkins

 

[indimech]

@ 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?

 

[Ussuri]

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.

 

[Ussuri]

736N, my mistake