English: Science Journal accelerometer data on a phone dropped and caught 3 times.
During the free-fall segments, the accelerometer reading drops to zero because gravity is a geometric force (like centrifugal) which acts on every ounce of the phone's structure, and is hence not detected.
This is because accelerometer's only detect (frame-invariant) proper-forces[1], like the upward force of your hand when holding the phone in place before the drop. The positive spikes occur when the fall is arrested as the falling phone is caught (also by hand) before it hit the floor.
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