Inconsistent behavior with stick to track disabled.

[bryo5363]

While testing issues I noticed that there is inconsistent behavior when the stick to track feature is disabled. The most clear demonstration of the inconsistent behavior is using a loop in the playground mode.

Steps to reproduce
-open sim
-click on playground mode
-create a loop
-disable stick to track
-drop the skater from the top at slightly different heights

[bryo5363]

I believe this issue is somehow related to #172. Some of the behavior involves the skater falling through the track or spawning on the other side of a track.

[samreid]

I created the track shown above and was able to demonstrate different behavior when dropping the skater from different heights. Sometimes he will pop out of the loop, and sometimes not. I commented in the commit above about the code that causes the skater to sometimes pass through the track when at shallow angles: the problem is that it is searching for a spline point nearby to the skater's initial location instead of halfway between the initial location and final location, but switching to search based on the centroid causes worse (buggier) problems.

[samreid]

Reassigning to @ariel-phet to discuss the severity and priority of this issue.

[ariel-phet]

I think since this behavior depends on a sensitive dependence to initial conditions it is OK for the moment.

[samreid]

@bryo5363 said:

@samreid @jbphet and I were talking about #194 and I believe this issue should be dealt with. Every time I create a loop with stick to track disabled, the skater will shoot through the track. If one had to create a very difficult path in order to reproduce this issue it would not be much of a problem but because the icon for the playground itself is a loop I believe people will be trying to recreate the track and experience this issue. Let me know what you think.

[samreid]

This behavior looks like it is significantly improved after addressing #207. Should be verified by QA.

[bryo5363]

@samreid I am currently testing on an iPad and the issue is still present when creating a loop.

[samreid]

What is the issue exactly? Can you produce the same problem on Win8/Chrome? Can you make a screencast about it?

[bryo5363]

When creating a loop with stick to track disabled, the skater will occasionally skate through the loop. Pressing return skater will replicate the exact motion but dropping the skater from a different position will create a new point where the skater will fall through.

[bryo5363]

I reproduced it on OSX 10.8.5 Chrome

[samreid]

In my experience testing the last few versions, I have seen the skater "skate through the loop" but on further inspection, the skater is going off of a tiny jump and moving beyond the track. It would be good to see the situation you are describing to see if that is the case.

[bryo5363]

@samreid I noticed this issue on 10.9.4 Safari 7.0.5. I created the same type of track that I have posted on this issue. I do not think that the skater is going off of a tiny jump; I dropped the skater from a higher point and did not observe the skater falling through but at a critical point the skater exhibits this behavior.

[bryo5363]

Also the skater did not fall through the loop until she completed a full completion of the track; it occurred the second time she went around.

[bryo5363]

I did not notice the issue on Safari 9537.85.10.17.1 but I did notice that the skater would still fall through the loop at various heights on Chrome 37.0.2062.120 on 10.9.5. I will test this issue on other devices before closing the issue.

[bryo5363]

Just noticed the issue is still present in the latest Safari. I created a track like this

The skater would usually go through the loop but occasionally would hit the end of the loop and bounce back up.

[bryo5363]

Here is another example of a track that will produce inconsistent behavior. When dropping the skater from a certain height, she will fly through the track when she hits the bottom.

This is the same track from my post in #297

[bryo5363]

I also noticed some strange way to produce this behavior. Drop the skater from the top of the track (try to recreate the track I posted above). Switch tabs immediately, wait about 3 minutes then switch back. The skater will always fly off the track when hitting the point of least PE

This behavior was noticed on OSX 10.9.5 Chrome 37.0.2062.120

[samreid]

I also commented on this issue in #297

[ariel-phet]

I would be concerned if this were happening with "stick to track" enabled. But since stick to track is disabled, I think some funkiness is allowed. In the real world to skate around a look generally requires a pretty smooth loop. I think we can leave as is. As long as energy is conserved in this situation we are OK. As much as I would like the behavior here to be perfect, we must remember that the goal of this sim is to focus on energy conservation, so a little bit of odd behavior is acceptable as long as Energy is still behaving properly.