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Comment by colanderman | Hacker News Reader

Maybe I'm missing something, but what kind of units are "foot pounds per square inch"? That's definitely not a pressure...

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Foot pounds are a commonly used unit of energy when discussing small projectiles such as bullets, pellets, arrows and bolts. I believe the author's use of "foot pounds per square inch" was an error.

WolframAlpha calculates the energy of a 40 grain bullet at 1550 feet per second to be 213.3 foot pounds, not 324. Winchester claims that a 40 grain .22 magnum bullet from a rifle-length barrel exits the muzzle at 1910 feet per second and has 324 foot pounds of kinetic energy. WolframAlpha agrees.

A foot pound is the amount of force required to move a pound of material one foot. By adding a surface area it becomes a measure of pressure.

A foot pound is the amount of energy used when applying one pound of force over the distance of one foot (specifically, with the direction of the force being the same as the direction of movement).

So, if you use one pound of force to move an object one foot, you have used one foot pound of energy (or about 1.355 Joules).

Pounds per surface area is pressure. Foot pounds per surface area is something else.

Any amount of material can be moved any distance with an infinitesimal amount of force, given enough time... or maybe I'm missing something.

It's the amount of energy required to lift one pound vertically by one foot, or the amount of momentum a one pound object gains by falling one foot in a vacuum.

Much of the confusion here comes from the fact that a pound is used both as a unit of force and as a unit of mass.

This is one of the many reasons y'all should use SI.

Not quite; friction has it's say first :)

Energy per area? A foot-pound is a unit of energy, analogous to a Joule, defined as the amount of energy required to apply one pound of force over a distance of one foot.

I can see a use for such a unit; it's just not one I've ever encountered before.

Well, a watt per square meter is a measure of irradiance, so I guess if you integrate that with respect to time you'd get joules per square meter. But I don't know if that gives you anything interesting.

The use I had in mind was for the energy cost of producing a material of uniform or irrelevant thickness, such as aluminum foil or photovoltaic cells.

Foot-pounds is very commonly used to measure firearm projectile energy; I've never seen foot-pounds per square inch before, though.

It's force per unit area, sounds like pressure.

Uh, isn't a pound-per-square-inch a pressure? A foot-pound-per-square-inch is a rather odd unit of energy.

Yes, a foot-pound per square inch would be a unit of torque per area. Maybe the bullet twisted its way in.

Not exactly. Here, foot-pound refers to the energy of the bullet, which can be calculated by taking the dot product of the force and the distance, where the force and distance refer to how far the bullet could exert some force. Taking the dot product means that only the components of the force and distance that are in the same direction are used to calculate the result.

When calculating torque, the force is applied in a direction perpendicular to the distance that is measured, because the distance refers to the distance between the place where the force is exerted and the center of rotation. Therefore torque is calculated by taking the cross product of the force and the distance. So while the units look the same, what they are measuring is different.

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