# Moments - engineering question

Discussion in 'Math' started by shortbus, Feb 21, 2011.

1. ### shortbus Thread Starter AAC Fanatic!

Sep 30, 2009
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I hope this is where to ask - In figuring a moment of force, if something will withstand 100 pound/foot, does that mean that it will withstand 200 pound/at 6 inches? Or is my thinking off here? Thanks for any help on this.

2. ### beenthere Retired Moderator

Apr 20, 2004
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100 pounds per foot (actually, square feet) is 100 pounds over 144 square inches.

200 pounds over 36 square inches is going to be the same as 800 pounds over 144 square inches.

Jul 7, 2009
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Your pound/foot units are wrong. Moment is equal to a torque, which is the vector product of position and force. Thus, the units should be length times force.

Note that you can't simply multiply the two numbers (magnitudes) together -- the vector product also involves the sine of the angle between the vectors. If the force was at right angles to the distance from the center of rotations, that's when you can multiply the magnitudes (because the sine of the angle is 1).

beenthere, your units are mass per unit area, as a pound (unless otherwise stated) is a unit of mass. Unfortunately, it is often assumed to be a force too (which causes confusion for beginners), in which case it should really be stated as "pounds force" or lbf for short. If you meant it as a force, then your units were for pressure.

4. ### shortbus Thread Starter AAC Fanatic!

Sep 30, 2009
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I must not be using the right terms again This is a problem of force to move a load on a rail. Or rather the force that the Linear bearing will take before coming of off the rail. Moments of force was the closest I could come up with. The catalog called it a moment, but it's similar to but not torque. Been away from this stuff too long. Thanks.

5. ### beenthere Retired Moderator

Apr 20, 2004
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Should have been in PSI. Some rust on the brain.

6. ### shortbus Thread Starter AAC Fanatic!

Sep 30, 2009
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For years it was called 'foot/pounds' but for some reason it has changed in most literature to 'pound/foot' Some thing to do with the European scientific notation. We won't change to Metric, but will change certain notations.