What? An accelerometer does measure acceleration, but relative to the free-fall acceleration.
That's different than measuring the change in acceleration.
I got it wrong in my first sentence - in order for an accelerometer to be used, there has to be changes in acceleration proportional to distance travelled. Per the rest of the discussion, there isn't any.
Re:Military technology: Magic bullets
by robotresearcher on Sunday January 15, 2012 @06:32PM
(#3468632)
You still have it wrong. An accelerometer measures acceleration.
Users usually calibrate the device at rest to get a good estimate of G so that it can be subtracted from accelerations measured later. Is that what you mean?
No. A falling object is at "0G", it is "weightless" (in a "drop seat" type carnival ride, when the ride triggers, you have no force on your butt). A bullet is a falling object after it leaves the gun. You cannot measure earth's gravitational attraction from the bullet's frame of reference because it is 0. Therefore, you cannot use earth's gravitational attraction to measure the bullet's spin rate because the value is zero, and zero has no direction information.
Re:Military technology: Magic bullets
by robotresearcher on Tuesday January 17, 2012 @07:58PM
(#3477309)
Sure, once the bullet is moving ballistically you can't measure G. My argument was with the statement that accelerometers measure changes in acceleration. They don't.
When the round is fired, it will experience a huge acceleration from the propellant along its axis plus a very large rotational acceleration as the rifling spins the round from 0 revolutions per second to a large number of revolutions per second.
Once the round leaves the barrel, it is falling at a constant acceleration (9.8m/s, freefall, 0G, "weightlessness"), so it will not be able to measure the earth's gravitational force. It also will be spinning at a constant rate because there is no outside force increasing or decreasing the spin rate, thus no rotational acceleration. While there will be acceleration due to the centripetal effect of the rotation, that acceleration is constant and thus not usable with an accelerometer for counting revolutions.