Can somebody please clear this up? A couple of posters have stated that 'pound' is a measure of force, and that it has gravity already 'built' into it.
You don't measure a rocket engine's reaction force in kilograms
do you? You'll hear it referred to as pounds of thrust
. In SI units it's the newton
You'll often hear that someone weighing "180 pounds" on earth will weigh but "30 pounds" on the moon. You don't hear about them losing 70 or so kilos, though, do you?
I can't help it that sloppy terminology has entered common language.
In the OP a weight of "4000 pounds" was mentioned and you had to calculate a potential energy PE=mgh. Now, if the problem mentioned a mass of 1800kg then you would immediately multiplied by 9.8m/s^2 to handle the "mg" term. But in "pounds", would you have multiplied by 32fps^2 to handle "mg" (staying in english units)? No.
Observe you suggested dividing by 2.2 and then multiplying by 9.8. But what were the units of that "2.2"? They were "pounds per kg". And if you carefully backtrack the units of PE=mgh using the conversion factor you will find that "pounds" aren't really a unit of mass since there's an acceleration built in.
lb*ft = M*g*h
on the right substitute g=m*s^-2, h=m
on the left ft=c*m (conversion factor "c" feet per meter)
solve for lb and you get something in the form
lb = (M*m*s^-2)/c
which is essentially F = Ma