Kilograms (Kg) and Kilograms-Force (Kgf)

[N_Molson]

It may seem very simple but I want to be absolutely certain of this :

Can I say that a model rocket with a mass of 1 kilogram and a thrust of 1 kilogram-force has a Thrust/Weight ratio of 1:1 ?

(at liftoff (T+0), practically the mass will lower as the fuel is burned).

 

[Urwumpe]

Yes.

 

[C3PO]

Is there a 'standard' gravitational acceleration that's used in this weird unit to convert it from the correct unit for force: Newton?

 

[Fabri91]

Yes, the usual 9.81.
On Earth (meaning surface gravitational acceleration of 9.81 m/s^2) an object which has a mass of 1 kg has a weight of 9.81 N, or 1 kgf.

The kilogram-force (kgf) much like the pound-force only (appears to) make sense when used with the standard gravitational acceleration we have on Earth: if they were defined on the Moon, for example, they would be quite a bit different.

 

[sputnik]

There is of course no such unit as "kilograms force". A kilogram is a unit of mass. A Newton is a measure of force. It should not be possible to confuse the two, which is a major advantage of SI. Unlike, say, some other systems, where the "multiply by 32.2 error" is all too common. And it would be worse in SI, since it looks a lot like a simple decimal shift.

 

[C3PO]

Yes, the usual 9.81.
On Earth (meaning surface gravitational acceleration of 9.81 m/s^2) an object which has a mass of 1 kg has a weight of 9.81 N, or 1 kgf.

The kilogram-force (kgf) much like the pound-force only (appears to) make sense when used with the standard gravitational acceleration we have on Earth: if they were defined on the Moon, for example, they would be quite a bit different.

How 'standard' is 9.81 m/s^2? If I 'ask' my TI-nspire I get 9.0665 m/s^2. I think that one doesn't include the rotation of the Earth.

IMHO those kind of units produce no advantage what so ever, unless you are trying to confuse somebody.

 

[Fabri91]

How 'standard' is 9.81 m/s^2? If I 'ask' my TI-nspire I get 9.0665 m/s^2. I think that one doesn't include the rotation of the Earth.

IMHO those kind of units produce no advantage what so ever, unless you are trying to confuse somebody.

Hm. I assumed it was standard since that is what I've always seen referenced to as gravitational acceleration at sea level.
Here ya go: http://en.wikipedia.org/wiki/Gravitational_acceleration#Gravity_model_For_Earth

But you're right, and these units are doing a fine job of confusing us.

 

[N_Molson]

Well, in fact I was searching for the way to calculate the Thrust/Weight* ratio, and I found on a site in french that you had to divide thrust in kilograms-force by weight in kilogram.

*The french translation of thrust is "poussée", weight is "poids" and mass is... "masse", so at least I'm sure I'm translating correctly. Thrust/Weight ratio is "Rapport Poids/Poussée" (notice the nasty inversion between thrust and weight, which doesn't help !!).

So how should we do it the SI way ? Convert weight (in kg) into mass (in N) ? Then dividing by the thrust (also in N ?). I'm very confused there.

 

[C3PO]

So how should we do it the SI way ? Convert weight (in kg) into mass (in N) ? Then dividing by the thrust (also in N ?). I'm very confused there.

I would do it that way, but swap the 'names'.

Weight: A force in N
Mass: Mass in kg

So that would be: Convert mass (kg) into weight (N), and then divide by thrust (N).

 

[Loru]

I prefer thrust to weight ratios calculated in N/kg which gives me acceleration in m/s2

 

[jarmonik]

There is of course no such unit as "kilograms force". A kilogram is a unit of mass. A Newton is a measure of force. It should not be possible to confuse the two, which is a major advantage of SI. Unlike, say, some other systems, where the "multiply by 32.2 error" is all too common. And it would be worse in SI, since it looks a lot like a simple decimal shift.

As far as I recall there is an unit of force 1 kp (kiloponds) = 9.80665 N. Not very common these days.