[granpa (OP)]

I created the following to help people understand the basic ideas of relativity.

The bottom part shows the same interaction shown in the top part but from the point of view of a rocket moving toward the left at 0.999c. When checking the numbers be careful not to do too much rounding off. Some of the numbers are extremely sensitive to even very tiny changes.

https://commons.wikimedia.org/wiki/File:Relativistic_Energy_and_Momentum.png

Image is public domain