Re: [Phys-l] force or mass

[John Denker <jsd@av8n.com>]

On 11/07/2008 02:35 PM, Philip Keller wrote:

> But back to the force or mass question:  I think I am missing
> something.  So if you define a pound as a unit of mass, and then
> observe one pound of mass accelerating at 32 ft/sec2,  then what is
> the magnitude and unit of the force responsible for that
> acceleration?  Do you still use F=ma?

This is discussed at
  http://www.av8n.com/physics/weight.htm#sec-units-laws

It says in part:

SI (Le Système International d’Unités) was designed so that most 
of the units are consistent with each other, and consistent with 
the laws of physics. For example, the SI unit of force is equal 
to the unit of mass times the unit of acceleration, so we can 
write F=ma without any conversion factors.

Other systems of units are not always so consistent, in which 
case you must re-think many of the laws of physics, with an eye 
toward putting in conversion factors where needed. For example,

    * If you want to measure acceleration in feet per second per 
second, mass in lb, and force in lbf, then you cannot write Newton’s 
second law in the form F = m a. Instead you must write F = m a/g. 
This is what I usually do, on the rare occasions when I am forced 
to calculate with pounds at all.
    * If you want to measure mass in lb and force in lbf, and still 
write F = m a without a conversion factor, then you must measure 
acceleration in Gees (not in feet per second squared). This is 
almost equivalent to the previous option; it just absorbs the 
conversion factor into the definition of acceleration.
    * If you want to measure acceleration in feet per second per 
second and mass in lb, and still write F = m a, then you must 
measure force in poundals.
    * Some older engineering books measure force in lbf, mass in 
slugs, and acceleration in feet per second per second. This allows 
them to write F = m a without a conversion factor.
    * My advice is to convert everything to SI at the first 
opportunity, so we can forget about ugly things like poundals 
and slugs.