Help with force / weight units...

[vcps2021]

Hello,

 

A structural engineer I am working with goes between using kg and kg/m2 and I wondered if anyone could help me understand the difference and whether they are equivalents.  In one location he says it's fine to use a pocket door of up to 60kg (so far so good) but in an other location he says it's not ok to use 60kg per m2 unless the floor is opened up to check joists. Is 60 kg per m2 like saying a door that weighs 60kg? Thanks for any help you can give - he doesn't seem too keen to answer this question - I guess he is more used to dealing with architects than homeowners but still....

 

Thanks!

[MikeSharp01]

One is basically weight 60Kg or a load at a point. The other is more like spread load. If you imagine 30 2kg bags of flour in a bag that is 60kg. Now mark out a 1m x 1m square and then spread the 30 bags out across the square and that will be 60Kg per square metre. 

[vcps2021]

Thanks for your reply - I understand that as a concept with the bags of flour example but when translating that to the real world situation where the pocket door is measured in kg and it's not spread over a metre square, I don't know how I use that information to find a suitable door....

[SteamyTea]

A kilogram is a standard unit of mass, mass is really the sum of all the 'stuff' in it.

Weight is the combination of mass and a force applied to it. Usually Earth's gravity.

 

The unit of Force is mass multiplied by acceleration, with the SI unit N, for newton.

Worth remembering that acceleration is a change in speed, and/or direction. It is either positive or negative.

[saveasteading]

16 hours ago, vcps2021 said:

60 kg per m2 like saying a door that weighs 60kg? 

Expanding on @MikeSharp01.   If you bought a lot of doors that weighed 60kg each and propped  them up, around a room so that each one had 1m2 around it.....then that would be one door per m2  .....or 60kg/m2.

 

That is a lot of load. But you won't have these heavy doors spread around tge room

 

It is a very good question esp as we can get into bad habits of using shortcuts. So in conversation your SE might say kg/m2 the first time , but then abbreviate to kg after that. 

Also saying kilogrammes one time and kay gee another.

 

In writing it will always be more correct.

 

A pocket door is on runners with a moving load. Leave that to the SE.

 

 

As @SteamyTea says, we also mix weight and force. Sometimes carelessly.  So kg is weight and N (Newton) or kN is force from the weight. Don't worry about it. 

 

Keep asking your SE and also on here.

 

 

 

 

[SteamyTea]

6 minutes ago, saveasteading said:

So kg is weight

Or mass even.

 

Weight should really be banned, mass and force should be the only terms used.

 

Just think how much time I would save now if I had fallen asleep in my Physics lessons.

Edited May 31, 2023 by SteamyTea

[George]

16 hours ago, vcps2021 said:

in an other location he says it's not ok to use 60kg per m2 unless the floor is opened up to check joists.

 

Was this specifically in relation to pocket doors?

[vcps2021]

Thanks for everyone's input - it's very much appreciated.........

 

George - yep, it's specifically about a pocket door

[Temp]

On 30/05/2023 at 18:07, vcps2021 said:

In one location he says it's fine to use a pocket door of up to 60kg (so far so good) but in an other location he says it's not ok to use 60kg per m2 unless the floor is opened up to check joists

 

I'm a bit late to this thread but..

 

To calculate a force you multiply the mass by the acceleration due to gravity g = 9.81m/s^2 (or 10 if you want to make the calculation easier) and you get an answer in Newtons (N). 

 

So a 60kg top hung pocket door exerts a force of about 60 * 10 = 600 N or 0.6kN on the track it hangs from. This is a point load as it acts more or less in one place.

 

If you had a floor that you planed to tile with heavy stone that has a mass of 60kg/m^2 (60 kg per square meter) the extra force on each square meter would be 600N or 0.6kN.  If your floor had an area of say 20 square meters the total force on the walls would be 20 * 600 = 12,000 N or 12kN.  This is a distributed load as its spread over the area of the floor. 

 

[SteamyTea]

F = ma

 

Where F = Force in newtons, not Newtons, that was the man, Sir Isaac Newtons.

m= mass in kg

a = acceleration in meters per second squared, m.s^-2.

 

To really confuse, read this about weight.

https://en.wikipedia.org/wiki/Weight

 

[Temp]

2 hours ago, SteamyTea said:

Where F = Force in newtons, not Newtons

 

I always forget its lower case n for newtons but N when abrieviating.

[George]

19 hours ago, vcps2021 said:

Thanks for everyone's input - it's very much appreciated.........

 

George - yep, it's specifically about a pocket door

 

Probably just a typo but not sure which way.

 

Mind you, unless it's some sort of blast door (or maybe which thick glazing) it'll probably not weight more than about 30kg in total. 

[SteamyTea]

1 hour ago, Temp said:

 

I always forget its lower case n for newtons but N when abrieviating.

Except for the exception, when at the start of a sentence.