Acceleration measurment

[Hans-Werner]

Hi there  ,
while reading the sensor for acceleration I recognized some odd facts. The value for acceleration in z-direction easily rises to high numbers of over 20 and more. Am I wrong when I assume that simple dropping of my smartphone (by the way a Samsung S7, what is not important as I think) should give a constant value of 9.81 !? 
When it hits the ground I could accept the rise to higher values, but not while falling. 

For the 'acceleration with g' I have another question: Isn't this the simple addition of 9.81 to the experiment 'without g'. Because I wonder how it can be possible for a system wich does not change (on the floor lying smartphone) to measure any acceleration...

Greetings from cologne 
(Not yet much experience but already loving this app)

[Sebastian Staacks]

Actually, it is the other way round:
The accelerometer is a sensor which measures forces acting on a mass. You can imagine it as a small test mass, which is kept in place by a few springs (here is a nice short animation of a more realistic model https://www.youtube.com/watch?v=4kfzqZpttTA). When you accelerate the system, the mass will move against these springs because of inertia. But it will also move against the springs because of the gravitational force. Therefore those accelerometers always measure the acceleration of the device and the earth's acceleration as well. (When referring to the animation: Imagine, that you would tilt the sensor, so that gravity pulls the inner part to one side).

This is why every device has an accelerometer. Originally, people were not that much interested in the actual acceleration, but in the direction of gravity, so the device can recognize when it is rotated, so it can rotate the content of the screen as well.

Also, this is why the sign of earth's acceleration seems to be wrong (depending on how you define this). If the phone is resting on a table, gravity will pull the mass "down". But to the only way to pull down the mass due to inertia is by accelerating the phone up. So "downward" gravity has the same sign as "upward acceleration", which is somewhat unintuitive.

Now for the acceleration "without g":
This one is actually a bit tricky and is usually composed from the accelerometer with g and the gyroscope. The trick usually is, to assume that the measurement starts with a resting device, so you can just subtract the measurement at the beginning (the 9.81 m/s²). After that, you need gyroscope to keep track of any rotation, because if the phone rotates, you need to subtract these 9.81 m/s² from a different direction. On top of this, the sensors tend to drift a bit, so there usually are some algorithms that try to remove drift with additional assumptions.

Luckily for us, phyphox does not need to do this itself, but instead the "acceleration without g" is provided by the system (on both, Android and iOS). The downside of this is, that we cannot really know what the maker of a phone is doing there to generate this data and some phones even try to do these calculations without a gyroscope, which does not really work...

So, in fact, during a free fall, you would expect a constant 9.81 m/s² for the version "without g" and just zero for the version "with g". I have to admit, that I have no idea right now, how you could get 20 m/s²... Can you share data from such a drop?

(By the way, I just moved this thread to the general section.)

[Hans-Werner]

Thanks for the fast answer ...how the data is collected really is helpful for understanding...My phone still asserts being accelerated like a... lets say... a rocket.  here is the data. I held the phone at the height of my head and dropped it onto the bed
   

  FreeFall.txt (Size: 43.03 KB / Downloads: 650)

[Sebastian Staacks]

That looks just like expected

If you have a look at the data file, the drop starts at about 3.4s and the absolute acceleration almost immediately jumps to 9.8 m/s². The individual components (x, y and z) are a bit more complicated, but at the beginning, it is mostly z that has -9.8 m/s² (display pointing upwards during the fall), but which then slightly declines while x and y measure some increased acceleration as well. The total acceleration is still close to 9.8 m/s², so it seems like the phone rotated slightly during the fall (not much, something like 20°).

It is a bit difficult to read these numbers in the graph because the strong acceleration during the impact on the mattress dominates everything and scales the graph to the larger range of more than 60 m/s² on the z axis. In the upcoming big update, for which the test phase will start next week, you will be able to zoom in and pick individual data points, but at the moment it is hard to estimate the number. But in the raw data, you can clearly see 9.81m/s² for about 0.5s of free fall (this would match a drop height of about 1.2m) and then almost 80m/s² as the phone is stopped by the mattress in only 50ms. You can even see subsequent bounces after that...

[Roland Van Kerschaver]

https://www.google.be/url?sa=t&source=we...iEqmL3-zpe

An article in Physics Teacher 2012:
g + a = 3.5 g
a = 2.5 g

https://www.sciencelearn.org.nz/resource...celeration

Acceleration of rocket:
0.5 g
g = 9.81 m/s²

[Hans-Werner]

Thanks for your elaborate answers.  To recap: the recorded high acceleration of about 60 m/s^2 (btw: how to display superscripts?) during my free fall experiment came off through the impact on the cushion and was not a failure of my sensors/software measuring the g-force.

The issue with the rocket-acceleration: privily/in secret I have wished to get corrected (because I was unsure). Of course I was talking about a rocket wich accelerates near the sun towards its surface    

[Sebastian Staacks]

Yes, the 60 m/s² (or 80m/s² absolute) is just the peak of what you get when a mass is accelerated with 9.81m/s² for 500ms and needs to decelerate in a tenth of the time.

The forum does not have a plugin for formulas (yet? might be worth adding...). So, the "²" is just a unicode character, which I typed directly. On German keyboards it is [Alt Gr]+[2] (there is probably something similar on other layouts as well).

[Roland Van Kerschaver]

With my keyboard it was not possible to use supscript so I copied m/s² elsewhere on the forum and pasted it.