Bug with scene

[vk4]

Hi,

I done a scene where a ball turn alone even there is friction. This is an error known in Algodoo software ?

Thanks for your help

[electronicboy]

The big object has a high level of attraction, it's inducing magnetic pull, which because of the object being pressed against something, is being transferred into rotation.

Attraction inducing rotation

electronicboy

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See the scene, G is the force caused by the large magnet, which by means of the axle is transferring it into rotational movement.

[vk4]

For you it's logical the object in your scene increase its rotationnal speed alone ? this would say it can create energy, it's not possible in physics. Could you explain more please, I don't understand.

Note: your disk increase its rotationnal speed alone (I deleted all others things in the scene). I put attraction at 0 and it's the same, the disk increase its energy.

[Xray]

The rotation of the circle has something to do with gravity. If I disable gravity, then the circle no longer accelerates. If I enable gravity, but change its orientation from -90 deg. to 90 deg. then the circle rotates in the opposite direction. This is obviously a bug in the software.

[vk4]

True, and sometimes when you put friction from air this changes the direction of rotation. I thought the software compute sum of torque part after part and compute the sum after and like the walls cancel part of force (change with position) the sum is not 0. But it's possible to imagine a disk full with small balls, and this case can be compute and must give a torque =0, so it's not that.

[Xray]

I converted the polygon into a circle, and I still got the same result. BUT, if I add another circle and axle with exactly the same parameters, the new circle does not rotate by itself! So, there is something different about the original circle in the scene. I just haven't figured out what it is.

EDIT: I just realize that the circle will still accelerate on its own even with gravity turned off, but the acceleration is a lot less than with gravity turned on. CRAZY!

[vk4]

If you take perpendicular walls for stop the disk, it never turn around itself.

[Xray]

I also found that if I delete the axle, and then replace it with another one, the circle might spin differently, and when I resize the axle, I can get a certain size where the circle will oscillate forward and backward at a rate of about 3 seconds per cycle! WEIRD!

[Xray]

Check this out!

Weird circle behavior.phz

(281.29 KiB) Downloaded 149 times

[vk4]

yeah, and it's difficult to predict the rotation or not, I don't find parameters

[Kilinich]

Hinge just not in center of mass, that's all.

[vk4]

@Kilinich: ok, I understood your message, and you're right for the last scene, but this don't explain my first scene. Like this works with friction I thought it was the friction itself that give torque, look at image: X and f1+N1 give a torque in the contrary direction that the friction is, so it's a motor torque.

[Xray]

kilinich - That doesn't explain why the circle continues to accelerate! Where is that excess energy coming from?

[vk4]

The image showing all forces, Algodoo find force T, this force give energy, where does it come ?

[vk4]

I explained on image a possibility of error. If the disk turn on left wall and if there is friction force on top wall is lower and the sum of force give a torque on disk.

[vk4]

In this case the energie win with kinetic angular energy reach 50000 J in few minutes. There is a big error. The response is very linear, you can change the attraction of wheel or move the position of rectangle, this change the slope of rotational speed. Even the software compute part a part and sum after, this would say with a circle composed of a lot of part but separate, the disk would give energy...

If rectangle attract disk : NO ROTATION but if disk attract rectangle : rotation.

[vk4]

I can understand why with axle there is an asymmetric function. An axle has 6 holes, this is no matter in this place, so no attraction. You can test with a simple example, take an object an try to move around axle, this will move like a magnet. The function is very linear and works better with a linear attraction.

@Xray: this explain the see-saw magnitude of rotationnal speed

[vk4]

I tested with spring (fixed center not an axle) it's the same problem.