Question

Could someone explain force 'newton' with English system?

My physics professors have always used the international system for doing our physics calculations, which is great because I find the metric system a lot easier.

I live in the U.S. and recently though, my Cal 2 professor went into a section of application to 'physics' and he's using the English system !

Which confuses me because they use pounds. He said that a pound was a force, and thus foot pounds where work. If pounds are force then what do they multiply gravity by to get pounds as force? Force= Mass x Gravity, What do they use for mass?

So could someone clear up what the differences are for 'force' in the English system instead of the international system.

Thanks,

Answers

1 lbf= 1 lbm * g

g=9.8 m/s^2 or g=32.18 ft/s^2

hence

1 lbf= 1 lbm * 9.8 m/s^2

or

1 lbf=1 lbm * 32.17 ft/s^2

to move everything to SI units:

1 lbf=1 lbm * 9.8 m/s^2 and since 1lbm=0.454 kg

1 lbf= 1lbm * 0.454 kg/1 lbm * 9.8 m/s^2

1lbf= 4.449 kg*m/^2 = 4.449 N

did you see? they are different!

good luck!

Source(s):

http://en.wikipedia.org/wiki/Pound-force

#1

Ok, here's how it got explained to me.

Mass is a constant. Eg if you were to take 1kg of cheese into space, it would still be 1kg of cheese.

Weight, is not a constant, weight is relative. If you took that same 1kg of cheese into space, it would be weightless. ie 0kg

Yes, Force = Mass x Acceleration, which, when on earth Acceleration = Gravity

Because we tend to weigh everything here on earth, all scales are calibrated to tell the MASS of an object, in Kg (or pounds, or whatever), because as long as we are on earth gravity is (fairly) constant. Really, these weight units are incorrect, because we're not actually measuring the mass, we're measuring the force the object applies to the scale in earths gravity - that force would generally be measured in newtons. However because we assume gravity is going to be constant, we just infer the mass from something's weight.

The action that pushes the scale down isnt the mass, or the acceleration of gravity - its a product of both, the force.

I hope that makes some sense - the bit that confused me was that before i studied physics, i tended to think of weight and mass as the same thing, because i get told a block of cheese weighs 1kg. When really, it has a mass of 1kg, and we only kow that because we consider earths gravity.

#2

1 SLUG ====> The US Unit of mass

1 Pound = US Unit of FORCE = 1 slug*foot/s^2

F = MA

F = 1 kg * 9.8 m/s^2 = 9.8 Newton

F = 1 slug * 32.17 ft/s^2 = 32.17 pounds

#3

The English system is also known as the ft?slug?sec system. The slug is the unit of mass, analogous to the kg. The pound is equivalent to the newton; the weight of a 1 slug mass in pounds is m*g = (1.0)*32.2 = 32.2 lb at the earth's surface.

#4

It's just a matter of conversion. You need to memorize them I'm afraid. You need to memorize how many Newtons are in a pound of force for example, or how many feet are in a meter. Sad, but true.

It impedes learning the physics because you have to spend time learning the conversion rates when you could be learning the physics.

I hope your prof switches to the kms SI units once you all come to realize how silly the Imperial System, which is yet another name for the English System, is.

#5