# Question on "Orientation and Scale of Acceleration" intro

#1

On p 151 (of the electronic version) the second sentence in the section titled: “Orientation and Scale of Acceleration” says:
[quote](When the device is still, the accelerometer doesn’t know if it is moving at a constant velocity in the earth’s gravity well or if it is far out in space and accelerating upwards at 9.8 meters per second every second.)[/quote]

I’m trying to wrap my head around why that is. What is it about the device lying still that it could be caused by these two scenarios? And is it just these two? Are there others?

I’m sure I’m missing something obvious,
mb

#2

[quote=“Maroonblazer”]On p 151 (of the electronic version) the second sentence in the section titled: “Orientation and Scale of Acceleration” says:
[quote](When the device is still, the accelerometer doesn’t know if it is moving at a constant velocity in the earth’s gravity well or if it is far out in space and accelerating upwards at 9.8 meters per second every second.)[/quote]

I’m trying to wrap my head around why that is. What is it about the device lying still that it could be caused by these two scenarios? And is it just these two? Are there others?

I’m sure I’m missing something obvious,
mb[/quote]

Accelerometer is not measuring movement, it’s measuring acceleration. If the phone was in a train traveling at a steady 80MPH, it would show the same readings as if it were sitting on the table in your house not moving at all.

Acceleration is the “pull” the object feels. So, when you get in a fast car and stomp on the gas, you can feel your body being pulled into the back of the seat. Once the car is moving at a constant speed, you don’t feel like you’re being pulled into the back of the seat anymore. If you were holding your phone while the car was accelerating, you would see the numbers change, in effect measuring what your body is also feeling.

Gravity is a constant acceleration. The earth is constantly pulling on us (and on the moon… and on the international space station… etc). That’s why things, when dropped, go faster and faster until they hit the ground. (except for that pesky air resistance, which is pushing back the other direction) There is a famous experiment where an apple and a feather are placed in a vacuum (where there is no air resistance) and dropped. Both fell at the same rate of acceleration.

The accelerometer readings are in units of gravity. So, sitting flat on a table in your house, the phone will read approximately 1.0 towards the ground, and approximately 0.0 in the other 2 axes.

You can easily make the accelerometer read much greater than 1.0… just quickly move (accelerate) the phone with your hand. The acceleration and deceleration will reflect in the readings.

If you were to drop the phone in a perfect vacuum (where there is no air resistance / terminal velocity, etc), then the phone, just like you, would feel weightless and all 3 axes would measure approximately 0.0.

Hope this helps

#3

Thanks very much for the explanation. I should’ve taken more physics classes in school. Super clear now.