Imperial versus metric

Originally posted by scottishinnz
Well, considering that the mean depth of the ocean is only about 4 km, and the ocean trenches around 11 km deep, Verne obviously didn't know what he was talking about, and should be ignored.

Well, considering that the title referred to the distance travelled by the submarine under the sea, and not the depth reached, you obviously don't know what you're talking about, and should be ignored.

Originally posted by AThousandYoung
I think the idea is that the Imperial system is based on forces and not mass. Gravity is not the only force airplanes experience.

In metric forces are in newtons, whereas in Imperial you subscript pounds to be either pounds_force(lbf) or pounds_mass(lbm) the beauty of the system is that in imperial the distinction between weight and mass does not exist the way it does in metric.

For example if my weight is 70kg, then that is my mass on the earth. My weight is the reaction force my body experiences against the earth due to the gravitational acceleration my body experiences as a result of the force of attraction between two masses being proportional to the product of those masses and inversely proportional to the square of the distance between them.(all multiplied by a constant).

Because of the earths shape my weigh actually will vary over the surface of the earth and this is a function of the acceleration due to gravity changing over the surface of the earth. For all intents and purposes we accept g to be equal to 9.81m/s/s which means that as a 70 kg person my weight is 70X9.81=686.7N(newtons).

However my mass in pounds 70X2.2=154lbm also equals the force I exert on the ground 154lbf. If I want my mass in Imperial measure I need to divide my weight(154lbs) by 32.2 ft/s/s(the acceleration due to gravity in feet per second squared) to arrive at a value of 4.782 poundals, which is an Imperial measure rarely used.

The argument about Imperial and metric sized fastners and bolts is still probably the most convincing for manufacturers to maintain things the way they are. The retooling of Industry would go into the billions if not trillions of dollars as it is not just the US that is sized by Imperial dimensions.

In the aero industry at least, most of this discussion is moot anyway, notwithstanding the spectacular example of systems incompatibility that can lead to real objects vanishing(no I'm sure its over there!). By and large most critical performance data such as the Coeficients of Lift for various aerofoil sections and the Coefficients of Drag are numbers that have been non dimensionalised through their manipulation by other numbers such as Froude and Reynolds numbers such that an engineer in Europe talking of a lift coeficient at which stall occurs is directly comparable with an American engineers data,assuming that either had done their calcs in metric and Imperial respectively.

Originally posted by kmax87
If I want my mass in Imperial measure I need to divide my weight(154lbs) by 32.2 ft/s/s(the acceleration due to gravity in feet per second squared) to arrive at a value of 4.782 poundals, which is an Imperial measure rarely used.

This is not entirely true. If you want to turn the Imperial system into a fundamental system the way that metric is,involves the differentiating of mass and weight in the following way.

option 1

1lbf= 1 slug X 1 ft/s/s
and 1 slug = 32.2 lbm

option 2

1 poundal= 1lbm X 1 ft/s/s
and 1 poundal = 1lbf/32.2

option 2 is rarely used and I would be 4.782 poundals because my weight in lbf/32.2= 4.782. The division by 32.2 is not division by acceleration due to gravity so to assert that a poundal is equivalent as a mass unit in the way that a kg is a mass unit in the metric SI system is not strictly accurate. I would still be 154lbm.

This is slightly confusing because I would also weigh 4.782 slugs, though in all the above, the weight accorded my person is rather flattering, rather delusional and wildly innacurate.