APS Excellence in Physics Education Award
November 2019

Science SPORE Prize
November 2011

The Open Source Physics Project is supported by NSF DUE-0442581.

## Dynamic Rotation Measurement (Best practices) post and replies

How best to measure a rotating body that is also moving.
Paul Beeken
4 Posts

I know about how to track relative motion by creating a point mass and use it to move a dynamic coordinate system.1  I want to develop a conservation of angular motion lab and want to track the relative rotation of one object w.r.t. another.  More importantly I want to track the rotation of any moving object around it's center of mass.

What I have tried so far:
As an example look at the NTNU videos involving 2D collisions. The c.o.m. of each object is clearly visible.  I have defined a point mass for the c.o.m. of each puck. I have defined another point mass for the small offset dot, I can keep changing references to the c.o.m. to each puck and analyze the angular motion. Not too daunting a task but I was wondering if I am missing something obvious about this scenario that I might be able to use.

Is there a way to define an orientation vector between the c.o.m. of a puck and the corresponding offset mark?  Then I can simply chart the rotation? Would provide a nice visual.

1. We have Tracker 5.0 deployed on Linux machines in our classroom.

### Replies to How best to measure a rotating body that is also moving.

Re: How best to measure a rotating body that is also moving. -

Douglas Brown
339 Posts

If you create a tape measure then you can open the Attach dialog from its track menu.  Attach one end to the com and the other to the offset dot. The tape's data include its angle relative to the x-axis.

> I have defined a point mass for the c.o.m. of each
> puck. I have defined another point mass for the small
> offset dot, I can keep changing references to the
> c.o.m. to each puck and analyze the angular motion.
> Not too daunting a task but I was wondering if I am
> I might be able to use.
>
> Is there a way to define
> an orientation vector between the c.o.m. of a puck
> and the corresponding offset mark?  Then I can simply
> chart the rotation? Would provide a nice visual.>

Re: How best to measure a rotating body that is also moving. -
Paul Beeken
4 Posts

Perfect. That is much easier.  I'm writing it up now and wanted to include it in my 'hints and suggestions' section of the lab.  I was trying to figure our how to attach the protractor tool but that required 3 points.

Thank you.