{"action":"created","issue":{"url":"https://api.github.com/repos/tbeu/Modelica/issues/50","repository_url":"https://api.github.com/repos/tbeu/Modelica","labels_url":"https://api.github.com/repos/tbeu/Modelica/issues/50/labels{/name}","comments_url":"https://api.github.com/repos/tbeu/Modelica/issues/50/comments","events_url":"https://api.github.com/repos/tbeu/Modelica/issues/50/events","html_url":"https://github.com/tbeu/Modelica/issues/50","id":196681803,"number":50,"title":"Error in LineForce handling of potential 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'''3.0'''","creator":{"login":"modelica-trac-importer","id":22202507,"avatar_url":"https://avatars.githubusercontent.com/u/22202507?v=3","gravatar_id":"","url":"https://api.github.com/users/modelica-trac-importer","html_url":"https://github.com/modelica-trac-importer","followers_url":"https://api.github.com/users/modelica-trac-importer/followers","following_url":"https://api.github.com/users/modelica-trac-importer/following{/other_user}","gists_url":"https://api.github.com/users/modelica-trac-importer/gists{/gist_id}","starred_url":"https://api.github.com/users/modelica-trac-importer/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/modelica-trac-importer/subscriptions","organizations_url":"https://api.github.com/users/modelica-trac-importer/orgs","repos_url":"https://api.github.com/users/modelica-trac-importer/repos","events_url":"https://api.github.com/users/modelica-trac-importer/events{/privacy}","received_events_url":"https://api.github.com/users/modelica-trac-importer/received_events","type":"User","site_admin":false},"open_issues":1,"closed_issues":2,"state":"closed","created_at":"2016-12-20T13:08:42Z","updated_at":"2016-12-20T14:10:36Z","due_on":"2009-01-25T08:00:00Z","closed_at":"2016-12-20T13:08:42Z"},"comments":1,"created_at":"2016-12-20T14:10:36Z","updated_at":"2016-12-20T14:10:46Z","closed_at":null,"body":"**Reported by HansOlsson on 6 Mar 2008 08:13 UTC**\nNone\n\n----\n\nMigrated-From: https://trac.modelica.org/Modelica/ticket/50"},"comment":{"url":"https://api.github.com/repos/tbeu/Modelica/issues/comments/268252116","html_url":"https://github.com/tbeu/Modelica/issues/50#issuecomment-268252116","issue_url":"https://api.github.com/repos/tbeu/Modelica/issues/50","id":268252116,"user":{"login":"modelica-trac-importer","id":22202507,"avatar_url":"https://avatars.githubusercontent.com/u/22202507?v=3","gravatar_id":"","url":"https://api.github.com/users/modelica-trac-importer","html_url":"https://github.com/modelica-trac-importer","followers_url":"https://api.github.com/users/modelica-trac-importer/followers","following_url":"https://api.github.com/users/modelica-trac-importer/following{/other_user}","gists_url":"https://api.github.com/users/modelica-trac-importer/gists{/gist_id}","starred_url":"https://api.github.com/users/modelica-trac-importer/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/modelica-trac-importer/subscriptions","organizations_url":"https://api.github.com/users/modelica-trac-importer/orgs","repos_url":"https://api.github.com/users/modelica-trac-importer/repos","events_url":"https://api.github.com/users/modelica-trac-importer/events{/privacy}","received_events_url":"https://api.github.com/users/modelica-trac-importer/received_events","type":"User","site_admin":false},"created_at":"2016-12-20T14:10:46Z","updated_at":"2016-12-20T14:10:46Z","body":"**Comment by HansOlsson on 6 Mar 2008 08:19 UTC**\nWhen examining the LineForce I noticed this was similar to PointMass – and I remembered that we had to correct PointMass since it could lead to incorrect simulation results – basically if a pointmass was selected as root and something was connected to it was as if the point mass got an infinite rotational inertia.\n\nThus this got me worried; and, indeed, the LineForce has the same defect; I have attached two models that should behave very similarly – but they differ considerably in the result – since the frametranslation only is allowed to rotate in the ThreeSprings2 model –but not in the other model (you can also see that the frametranslation looks very odd in this model). My understanding is that this model is partially undefined – the frametranslation can freely rotate along its axis (i.e. one unspecified rotational degree of freedom – due to small inertia it is rotating very rapidly around this axis); whereas the original library model has 3 unspecified rotational d.o.fs.\n \nThe bug has been present since the early days of Modelica 2 (at least), and furthermore I cannot find any good correction of this. Doing as for PointMass would make it impossible to simulate the library example model ThreeSprings, and the documentation of ThreeSprings says \"This example demonstrates that 3-dimensional line force elements (here: Modelica.Mechanics.MultiBody.Forces.Spring elements) can be connected together in series without having a body with mass at the connection point (as usually required by multi-body programs). This is advantageous since stiff systems can be avoided, say, due to a stiff spring and a small mass at the connection point\"\n\nIt might be that this is a general problem with low priority potential roots."}}
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