Strange acceleration graph during a jump

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1. Sep 13, 2015

nickek

Hi!
I have just performed some experiments with my phones accelerometer, and plotted the acceleration as a function of time from the raw data. The experiments I did was:
• jumping down from a chair
• jumping from the floor and up
• let the device fall toward a bed
When I analyze the graphs, I think it's strange that the absolute value of the acceleration is more than g during the jumps, but close to g during the free fall (about 1.5g and 2.0g for jump from a 40 cm high chair and straight from the floor, respectively, during the free fall it seems). Can it be something I do during the jump (is technically possible without being stucked to the floor)?

During the jumping, the phone was fixed in the waist belt.

I attach the 3 graphs. Someone here who has an explanation for the marked strange parts in the graphs (it should be values of about -1, I think)?

2. Sep 13, 2015

Staff: Mentor

Sure. The position of your waist relative to your center of mass changes if you change your body shape, e.g. change the position of your legs (something you certainly do in some way during the jump), which leads to an acceleration.

How did you take into account that the phone measures all three acceleration axes? Its orientation might change during the jump.

I don't know smartphone accelerometers well enough, but in general those things need some time for a proper measurement (in the same way a bathroom scale will need some time for the measurement). If acceleration changes suddenly, the values might be off.

3. Sep 13, 2015

nickek

Thank you. Maybe the position thing. About the acceleration axes, I just used the z-values (positive upward).

4. Sep 13, 2015

Staff: Mentor

The orientation of the phone can change, which will influence the measurement.

5. Sep 13, 2015

CWatters

Try adding a line on the first graph at -1g.

6. Sep 16, 2015

UncertaintyAjay

It's also about frames of reference. When you're in an accelerating frame of reference, you feel an acceleration in the opposite direction to that of the reference frame. Say, for example you're in a very fast car( pick any one you like, Aston Martin DB9, personally). When you put your foot down on the accelerator, you'll get pushed back into your seat. This is because you are in an accelerating reference frame.( Reference frame is basically the system you measure things in. So, if you make physical measurements in an accelerating car, you're in an accelerated reference frame. This is interesting stuff, so Google it for more info.) That force that pushes you back into your seat is a pseudo force- the effect of the acceleration.
Pseudo forces are needed in order for Newtons laws to appear correct in your frame of reference. The thing about these forces that you observe them to act in a direction opposite to your motion. That explains the negative acceleration.
An important thing about pseudo forces is that you only observe them in accelerated reference frame. So if some external observer was measuring your acceleration by some method, he wouldn't get these weird data. Because your accelerometer was strapped to you and hence also being accelerated, you got these weird results.
Btw, good job on the experiments- that's some good physics.

7. Sep 16, 2015

Staff: Mentor

@UncertaintyAjay: Accelerometers at center of mass of an object in free fall will measure an acceleration of zero. As they expect to be on the surface of Earth, they subtract g and get -1 g acceleration in the lab frame. That's the baseline here. The thread is about the deviations from this -1 g.

8. Sep 17, 2015

UncertaintyAjay

Ah, but the reason an object in freefall feels weightless is because of the pseudo force in its frame of reference that's equal and opposite to the force of gravity on it. So maybe not that whole massive discussion but at least a small part of it is relevant to the topic right?

9. Sep 17, 2015