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There is something I don't understand about accelerometers. I know it's possible to make an Inertial Measurement Unit (IMU) by using "three" accelerometers. So with that, I could calculate the $x$, $y$, $z$ coordinates of something in 3-D space.

The point that I don't understand is, how could it sense constant-speed movements? When the velocity is constant, acceleration is zero. So how does it work?

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It can't directly(Einstein was right)

What you can do is integrate all the accelerations you have measured upto that point in time to have a current knowledge of the current velocity.

This isn't very accurate because any small inaccuracies in acceleration will lead to increasingly inaccurate velocity, and so position, with time - this is known as drift.

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What was Einstein said about this? –  Rckt Nov 4 '12 at 19:37
    
It's a fundamental principle of physics (dating back to Galileo) that you can't tell if you are moving without looking at something outside, and you can't tell if you are accelerating or in gravity. It's the basis of the theory of relativity –  Martin Beckett Nov 4 '12 at 21:03
    
@Rckt In short, that no laws of physics, and therefore no observations whatsoever, can determine an absolute velocity (or an absolute lack of any speed). All you can do is tell the relative velocities between two different observers' reference frames. –  Chris White Nov 4 '12 at 21:05
    
If this wasn't true then your lab experiments would give you a different answer in summer when you are going one way around the sun and int he winter when you are going back in the opposite direction. –  Martin Beckett Nov 4 '12 at 21:07
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