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iphoneiosphysicsaccelerometer

What exactly does the iPhone accelerometer measure?

The apple documentation for UIAcceleration class says,

"When a device is laying still with its back on a horizontal surface, each acceleration event has approximately the following values:
x: 0
y: 0
z: -1
"

Now, I am confused! How can the acceleration be non-zero, when you clearly say the "device is laying still"?

UPDATE

Judging by the responses, I think this should be called something like 'forceometer' or 'gravitometer' and not accelerometer!

Solution

  • I found the answer [in the CoreMotion Reference guide, thanks to bensnider:

    The accelerometer measures the sum of two acceleration vectors: gravity and user acceleration. User acceleration is the acceleration that the user imparts to the device.