So I'm upgrading my Physics Engine's Collision response system (don't worry, this is a pure physics question, no programming involved! Also don't worry about the time of collision, my collision detection system is separate and returns a time of collision for the response system to work with) and I found this really great guide: http://www.myphysicslab.com/collision.html The equation I'm struggling with is Vap1 = Va1 + (ωa1 x rap) Two questions: 1) The angular velocity is and has to be in radians/time_unit, correct? (Probably a stupid question, but I can't figure it out for sure and wanted to be certain). Or does it even matter as long as I'm consistent? Why? 2) How can you cross the angular velocity with the rap vector? Isn't the angular velocity a scalar? An example of how to accomplish the math would be greatly appreciated. Thank you in advance for your time and help!