Feature Showcase - Rigid Body Box

SpinningBox Example

As I mentioned in the blog last week, MOMDYN now includes its first rigid body component, the box. The idea is to create a rectangular prism with uniform density, length, width, and height dimensions. I included the example model "SpinningBox" in the import menu, which is the same as the GIF on this page.

The SpinningBox example is a nice demonstration of dynamic stability of rigid bodies. I can recall my professor showing this with a text book. Grab a book, you can experiment at home. Try holding the book by its two bottom corners, then throw it upwards and try to get it to spin around its horizontal (left-to-right). axis. Chances are, it will start to tumble rather than spin about the single axis. On the other hand, try flipping the book about its other two axes and they should not tumble. A quick explanation of this is here.

In the example, the body y-axis (ey) is set to have the intermediate moment of inertia, and is therefore unstable. The simulation is initiated with a 2.0 rad/sec initial angular velocity about ey, and a smaller rate of 0.2 rad/sec about the body x-axis (ex). You can see the instability, or tumbling, in the GIF; the ey axis starts pointing up and to the left, then the body flips after about a half rotation until ey points down and to the right. After another 2 rotations, the ey axis flips back to its original position.

Suggest experimenting with the example, for instance:

• In the generalized speeds menu, click the editbutton, make ω_x large and ω_y small
• In the rigid body box menu, click the edit button, change length, width, or height dimensions

Enjoy.

Whats new in version 0.5?

As of yesterday (3/15) I have submitted a new version of momdyn to App Store Connect for the Test Flight beta, and to the Google Play Store for alpha testing. Thats right, finally compiled a version for Android! Besides that, here is a quick summary of whats new in this release:

• New solvers, including with adaptive step sizing
• New rigid body box
• Simulation status is displayed with completion ratio and time step
• Report view is split into pages for each component type