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Bad values of g with phone camera post and replies

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Bad values of g with phone camera - Nov 18 at 7:27PM
JULIAN ORO
2 Posts

With the phone cameras (Huawei, android system, 30 fps) I can't get good values of g in a free fall, I get higher values 10.8 or 11. With my old video tape camcorder  I get good values, someone knows how to solve the problem?

Post edited November 19, 2018 at 6:54 AM EST.


Replies to Bad values of g with phone camera

Re: Bad values of g with phone camera - Nov 19 1:32PM
Paul Nord
18 Posts

Jullian,

There are lots of ways to get the wrong value with video analysis.  A 10% measurement might be pretty good depending on the setup.

Can you post a video of your experiment?  This will answer a lot of questions.

The first question is of course, is the camera being held steady?  If your frame of reference is moving, don't expect the observed motion to be newtonian.

Second is whether the object being measured is at the same distance from the camera as the scale object you are using for a distance reference.  Since the camera is projecting a 3 dimensional world onto two dimensions, objects that are closer to the camera appear larger and their motions are bigger in relation to objects in the background.  If your scale is a stripe painted on a wall, an object moving in front of that wall will appear to be moving farther and faster in proportion to its distance from the wall and inversely proportional to the distance to the camera.  (e.g. camera 1 meter from object, scale 0.1 meters behind object, the object size and velocity will appear 10% larger than it really is.)

If you can't account for a systematic error with those, there are some more subtle video problems that have to do with motion toward or away from the camera, rolling shutter, and even erroneous frame rates.

Paul



Re: Bad values of g with phone camera - Nov 19 2:50PM
JULIAN ORO
2 Posts

Paul,
Thank you for your answer, related to the questions you ask me, I can give you some answer.
The camera remains fixed on a table and vertical, the ball that goes up and down is at a distance from the reference object of 10 cm, but the camera was perhaps too close to the ball, which gave a higher speed and did not compensate the distance to the scale. I filmed a little bit further at 7 m and the result was better..
Another problem that I think exists is that with the mobile phone's cameras  there are erroneus frame rates, which is not the case with video cameras. I have filmed with a video camera and the result it's better also.

Thank you for your answer.

Julian



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