What's (lbm) and (lbf)?
Donald G. Shead
~ I'd Like to Know ~
What, exactly, is the difference between a pound mass (lbm), and a
pound force (lbf)? Does anybody know?
Donald Shead <u10...@snet.net>
Ken Fischer
Donald G. Shead (u10...@snet.net) wrote:
: ~ I'd Like to Know ~
: What, exactly, is the difference between a pound mass (lbm), and a
: pound force (lbf)? Does anybody know?
In a 1 g gravitational field, or on a space ship in
outer space accelerating at 1 g, not much, except that mass
refers to the resistance to acceleration of a defined
quantity of matter.
If you want me to go into the units of pounds,
poundals, Newtons, etc., sorry, I don't want to.
I am content to consider 1 pound of force to
be that which will accelerate 1 pound mass at 1 g.
There is a problem with this in advanced physics,
but I never do anything advanced. :-)
Ken Fischer
Ron Thomson
> What, exactly, is the difference between a pound mass (lbm), and a
> pound force (lbf)? Does anybody know?
You first have to be clear about the difference between mass and
force. There are various levels at which these can be defined.
At high-school level you might say that force is something which
acts on a body's mass, while mass is the quantity of matter on
which the force acts. Mass is a scalar, force is a vector.
The key is Newton's equation:
Force = mass * acceleration.
You can then set about defining units. This is usually (but not
necessarily) done by thinking of a mass suspended or falling under
gravity. Suppose we talk in SI units, since these make a clearer
distinction between mass and force. The SI unit of mass is the
kilogram and gravitational acceleration g is almost 10 m/s**2.
So a mass of 1 kg experiences a gravitational force of 1 kg *
10 m/s**2 = 10 units. This force is commonly called the weight
of the body. The SI force unit is called a Newton.
Things become more complicated in the old Imperial units. Suppose
we suspend or drop a 1 lb mass in a gravitational field. The
acceleration is about 32 ft/s**2. The weight of the body is then
1 lb(m)* 32 ft/s**2 = 32 units of force. The unit of force here
is called a poundal.
Alternatively we can choose to measure force in lb, as lbf. The
mass unit then has to be force/acceleration. This mass unit is
called a slug (really!).
Just to mess things up fully, you could choose to measure mass in
lbm and force in lbf but that's equivalent to using an acceleration
of 1 ft/s**2 in Newton's equation or normalising the gravitational
units with respect to g.
Regards,
Dr Ron Thomson
Ballistics + Impact Group
Glasgow University