Pounds is a unit of mass or weight

[D H]

Then if you take into account the local geology and the rotation you get more variation -
The average is 9.80665 m/s²

That 9.80665 m/s² is purely definitional and is an exact figure (something you only get with definitions). It is not an average value. It is what the value would be at Paris were Paris at sea level and if the Earth had a uniform mass distribution, truncated to five decimal places.

 

[mgb_phys]

That 9.80665 m/s² is purely definitional and is an exact figure

Sorry loose wording - it's an accepted aggregate value, not strictly an average.
It's actually aimed at 45deg N, not Paris (which is near 49N and has g=9.809)

 

[Dadface]

Pounds slugs?What about dynes ,ergs and other units?Physicists around the world started to phase these units out on the early 1960s.

 

[binoyjjohn]

Dear all,

I know that this is a very old post.
I am also bit confused, since the imperial system of units are not consistent, or, are not used as it should be.

Here's the difference: -

In Metric system, or SI system, Mass (kg) is a fundamental unit and Force is a derived unit (N), that means, force is derived from mass, so when you multiply the fundamental unit, kg with acceleration, we get the derived unit, N (F = Ma).

In imperial system (or British or American) system, the Force (called as pounds or pound-force, lb or lbf) is the fundamental unit, and mass (lbm) is a derived unit, so when you divide force (lbf) with acceleration (in/sec^2) we get mass (lbm).

In actuality, one should never use lb as a unit of mass, it should be lbm (pound-mass).

Proof:

Metric system units

Mass: kg
Acceleration: m/s^2
Force: N or kg.m/s^2

Imperial system units (I am considering IPS system here)

Force: lb or lbf
Acceleration: in/s^2
Mass: lb.s^2/in

People have become so lazy that the standards are not followed properly.

Thanks,

Binoy John.

 

[D H]

Dear all,

I know that this is a very old post.
I am also bit confused, since the imperial system of units are not consistent, or, are not used as it should be.

Your "proof" is incorrect. English units are not consistent. That's the way it is. Then again, SI units aren't consistent, either.

Prior to the development of the metric system, Newton's second law was of the form F=kma, where k is a constant of proportionality. It is important to remember that Newton said that force is proportional to the change in momentum. He did not say F=ma. The English system explicitly maintains this constant of proportionality. In SI units, the force unit was chosen to make this proportionality constant have a numeric value of 1. While it is a pain in the rear that the proportionality constant is not 1 in English units, it is not incorrect. It is just "inconsistent."

However, we still write Newton's law of gravitation as F=GMm/r². The G in this expression is a constant of proportionality. A fully consistent set of units would have units of length, mass, and time such that G has a numeric value of 1.

 

[binoyjjohn]

Hi DH,

Is it really important to talk about F=ma, F=kma??
What is the unit of G? Let G be infinite to the power infinite, but still a constant. Will it make a difference to the unit of Force, mass or acceleration??
I believe that our topic was about the consistency in usage of units.

Thanks

Binoy

 

[D H]

I believe that our topic was about the consistency in usage of units.

It is bad form to drag a thread off-topic or, for that matter, to drag an old thread back from the dead. Thread closed.The topic of this thread is whether a pound is a unit of mass or a unit of force. The answer is yes. End of story. Thread closed.