Your completely correct, and that's at the top of my list for improvements. The lack of a charging/firing indicator makes Cathy very sneaky.
The physics system grew quite organically into it's current form. As we did early testing it became apparent adjusting the mass of objects was all we needed to do to tune things. Their mass is essentially their hit points, I thought about making "health" indicators but time did not allow.
Cathy simply weighs more than everything else, but, but she can be worn down. I'd guess it takes about $10,000 to do it.