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    <p>To solve this, if possible, you would need to reference something <strong>external to the car</strong>. Probably using the <strong>GPS</strong> to measure elevation gain is the most direct approach. That is, calculate the slope of the hill using the measured elevation gain, and use this to compensate the acceleration measurement.</p> <p>As others have mentioned, at a very deep level, there's no way to distinguish between gravity and acceleration; this is known as the <a href="http://en.wikipedia.org/wiki/Equivalence_principle" rel="nofollow noreferrer">Equivalence Principle</a> (and was one of Einstein's most important insights). So one would need to know more than the accelerometer's output to solve this problem, and this makes the problem difficult. For various approaches to the problem, see <a href="http://en.wikipedia.org/wiki/Inertial_guidance_system" rel="nofollow noreferrer">here</a>. </p> <p>Your best hope is to reference something external to the car, like a GPS signal, or pictures to measure the change in the location of something outside the car, or the earth's magnetic field, etc. Since this will take place near a lot of metal (the car) the earth's field will likely be difficult to measure, and continuous photos of some reference object (like the sun) is obviously difficult, so it seems that the GPS is your best bet.</p> <p>More on why you need an external reference:</p> <p>It's worth considering whether you could use the unique features of gravity that 1) it's always “on”, 2) it always has the same magnitude (for this situation). But when all you know is the total acceleration, you can't reliably separate the two contributions. As the picture below shows, just knowing the magnitude of gravity isn't enough... the picture below is drawn in the phone's frame of reference where it measures the resultant acceleration (black vector), and the larger colored arrows are possible gravity vectors, while the matching color smaller arrows are the corresponding acceleration vectors. So you can see, without knowing and angle, you don't have enough information to solve the problem.</p> <p><a href="http://i42.tinypic.com/6gasnb.png" rel="nofollow noreferrer">alt text http://i42.tinypic.com/6gasnb.png</a></p> <p>The other thing you might be able to do is integrate the output from a gyro to find the angle of the gravity vector. That is, assuming that you start on the flat, and then you always keep track of the where you think gravity would be based on the accumulated small changes. But this is highly error prone, and your error in the calculation will accumulate quickly with time; and it also assumes that you have access to a gyro.</p> <p>Therefore, the only good solution is to use external information.</p>
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    1. PODetecting Acceleration in a car (iPhone Accelerometer)
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    1. COTom, gravity *is* acceleration.. no need to bring in Equivalence Principle or so...
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    2. CO@Soham – Gravity is acceleration *because* of the equivalence principle. If this weren't the case, then one could distinguish between the two using, for example, two accelerometers with two different masses on the springs, and probably all accelerometers would be built this way, and the OP's question wouldn't have been asked. It's only that inertial mass and gravitational mass are the same that you say “is” with such confidence. (Before the equivalence principle people used to think gravity was a force that caused acceleration, not that it was one.)
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    3. CODoes inertial and gravitational mass ever differ? I just felt, that quoting EP in such a situation was tantamount to overkill
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