Gravity is up?

What’s the first thing we’ll do?

Perform a sanity check!

The starter code prints the X, Y and Z components of the acceleration measured by the accelerometer.The values have already been “scaled” and have units of gs. Where 1 g is equal to theacceleration of the gravity, about 9.8 meters per second squared.

{{#include src/main.rs}}

The output of this program with the board sitting still will be something like:

$ # itmdump console
(..)
(0.0, 0.0, 1.078125)
(0.0, 0.0, 1.078125)
(0.0, 0.0, 1.171875)
(0.0, 0.0, 1.03125)
(0.0, 0.0, 1.078125)

Which is weird because the board is not moving yet its acceleration is non-zero. What’s going on?This must be related to the gravity, right? Because the acceleration of gravity is 1 g. But thegravity pulls objects downwards so the acceleration along the Z axis should be negative not positive…

Did the program get the Z axis backwards? Nope, you can test rotating the board to align the gravityto the X or Y axis but the acceleration measured by the accelerometer is always pointing up.

What happens here is that the accelerometer is measuring the proper acceleration of the board notthe acceleration you are observing. This proper acceleration is the acceleration of the board asseen from a observer that’s in free fall. An observer that’s in free fall is moving toward thecenter of the the Earth with an acceleration of 1g; from its point of view the board is actuallymoving upwards (away from the center of the Earth) with an acceleration of 1g. And that’s why theproper acceleration is pointing up. This also means that if the board was in free fall, theaccelerometer would report a proper acceleration of zero. Please, don’t try that at home.

Yes, physics is hard. Let’s move on.