Odd behaviour of Point on Circle when Rotated

Swulf shared this problem 9 years ago
New

Hello,


In the attached script I have affixed a Point (A) to a Circle (d') - using Point on Object - with the aim of rotating the circle about another point and having A move correctly with the rotation of the circle.


This does not happen - and further, point A 'jumps around' the circumference as I manually move slider f. (Try putting a Trace on A for example).


What is going on?


Geogebra 4.0.20.0.


Swulf

https://ggbm.at/553499

Comments (9)

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I cannot reproduce any odd behaviour in 4.0.22. Seems to work as intended.

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For me, the behaviour persists even under 4.0.22.0. :(


I attach a screengrab as evidence...


2172642df76596b3e8e2e4c4be9e8d89

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move correctly


What do you mean by "move correctly"?


As the circle moves, GeoGebra will try and keep point A as close as possible to its last position. If slider f is moved quickly, then Point A appears to jump.


I think we might be making this behaviour configurable in GeoGebra 4.2 - I'll ask Mathieu.

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What I expected to happen was that point A - attached to the circle that I rotate - would move around the circumference of the circle - because it is 'fixed' to the circumference (I hope) - as I move the slider.


Perhaps a better description: if I take a coin and draw a dot on its circumference somewhere, that is point A. I then rotate the coin around its geometrical center (this is what I try to do in my script) - I expect point A on the circumference to move as the whole 'coin' rotates.


I don't understand why the script doesn't exhibit that behaviour. It seems pretty simple.

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Hi,


I guess I am still not being clear enough.


I don't want to rotate the point A explicitly with a command that acts upon A directly, as you give in your examples.


Instead, I want to A to rotate BECAUSE it is ATTACHED to a point on the circumference of the circle I happen to be rotating. I want to observe the behaviour of the attached point on the circumference, not just rotate the point A around some center.

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For me it works quite OK and I don't see the jumping. Note that moving the slider with mouse has limited sensitivity -- the slider is 100pixels long => only 100 positions can be achieved. Try changing value of the slider using keyboard arrows instead of mouse to obtain smooth results.

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Hi,


If I change 'f' with the keyboard arrow keys, point A doesn't move at all. :(

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Sorry :flushed:


I was trying it in the development version which is different. I'm not sure if this behavior is already part of 4.2 beta. With 4.0.22 the point really stays at the same place (or moves randomly).

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hello

i think that point[d'] has a one degree random freedom. you can reduce this freedom doing

a ->slider from 0 to 1

A=Point[d',a]


then you can move the circle and 'a'


saludos

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