# [✓] Dynamic drop/impact simulation of a rigid bar

GROUPS:
 HI All, I am new to SM and I am trying to extend the solution of a two-dimensional, textbook dynamics problem. It is a rigid bar initially at some angle and dropped some distance above the ground. The bar falls due to gravity and one end contacts the ground. There is a rebound of that point and an angular velocity is also imparted to the bar. As the bar continues to rotate, the other end of the bar will contact the ground, rebound and there will again be a change in the angular velocity, etc.I have worked through the bouncing ball example problems etc. I have built a model using a bodybox as the bar. I have been trying to use the elastogap component to model the contacts with the ground. I have been using a revolute component and a prismatic component to interface with the elastogap component.The issue I am having is, when I add a similar set of components to the other end of the, the problem won't solve. I suspect the problems is over-constrained at that point. So question is how to adjust the components to get both ends making contact and the problem will solve? Also, in general, is there an easier way than the path I have started down?Thanks in advance George
11 months ago
7 Replies
 George,Can you post your model? Its hard to answer that question in the abstract. I believe the problem is when you add the "similar set of components" to the other side, you MUST set their stateselect (in the Advanced tab) to "never". Your state is fully determined by the first set of components. You can't add redundant components unless you tell WSM to never select them as states.I attached a model of a bar that falls under gravity and has an elastogap on each end-- I set the damping to zero so it will bounce around forever. Is this what you want? Notice all of the elements on the right (the upper end of the bar) have the stateselect to never. Attachments:
11 months ago
 Here is the model in Diagram view:
11 months ago
 hi Neil, thank you for your help. I have attached my model, which, looking at yours, seems a bit too simplistic . It had been a series of trial and errors on my model.I have tried to run your model and got the error at the bottom of this reply. I have tried adjusting the "states" for components on the RHS to "blank" and then again to "avoid" When I do this, the program compiles and runs, but ends up with a 'divide by zero' error. Do you know what might be happening here? Finally, none of this is really intuitive to a newbie. Do you have a good reference you can recommend for mechanics/dynamics problems in WSM?Thank you again for your help GeorgeBuilding "RodExample" as experiment "RodExample 3" started at 08:01:42 Error: Error: Deselecting states, too many variables have StateSelect.never (or StateSelect.default or StateSelect.avoid and does not appear differentiated) (in set: universal2.w_b). Build finished at 08:01:44 (took 00:01). Error: No executable generated C:/Users/gethiel/AppData/Local/Temp/WolframSystemModeler-4.3.1/sme.4.3.1150047650215724.exe Error: No settings file generated C:/Users/gethiel/AppData/Local/Temp/WolframSystemModeler-4.3.1/sme.4.3.1150047650215724_init.sim Attachments:
11 months ago
 George,Sorry, I forgot that I use a different setting. Go into Preferences->translation and disable dynamic state select and my model will run. Since we are specifying the states manually you need to turn that feature off.You can experiment by giving my model an initial position (for example, specifying a drop height by setting s to say 0.3 in the prismatic joint called lift1 and setting "fixed" to true.)Also you probably want to add a bit of damping (500 works well and still bounces). I'll look at your model now...Regards