Virtual mass is a pain in the ass to accurately but it clearly explains why lighter arrows are less efficient than heavier arrows. Virtual mass can also give you a way to measure histerisis in a bow. It works like this, you measure your arrow speed and weigh the arrow. Next you very accurately weigh the stored energy in the bow (FDC).
The next step you can use one of the kinetic energy calculators they have on line for. Here we see how much KE the arrow had example 500 grain arrow traevling at 170 fps has 32.054 Ft# of KE, next you use the stored energy figure and plug that into your calculator. You add weight to your arrow figure on the calculator until the KE matches the stored energy your bow has at the same 170 fps. Example bow has 48 ft# SE, a 750 grain arrow carrying 48# of KE will travel at 170 fps. I subtract 500 grains from 750 grains and I have a virtual mass of the bow that is 250 grains. On a wood bow a portion of that 250 grains is histerisis that is sensitive to bow speed so it Lessens the value of this number but if you make about a 5% allowance for histerisis you would have a virtual mass of about 238 grains. This will allow you to predict arrow speeds of different weights.
Suppose you wanted to know how fast a 300 grain arrow should travel, just add the 238 to the 300 grains and see how fast a 538 grain arrow would have to be traveling to give you 48# KE. It should be about 202 fps. Now look at the efficiency of the 500 grain arrow and the 300 grain arrow. You have 66% and 56% efficiency.
