The simple physics formulas are:

Ke = 1/2 (m) ( v^2) where Ke is kinetic energy, m is mass, and v is velocity

P = mv where P is momentum, m is mass, and v is velocity

Of course, if you are looking for units of (lb)(feet)/sec then make sure you use those units in your measurement or convert them accordingly.

Momentum is the more important factor because it reflects loss due to things like sounds and friction more accurately when considering penetration. Friction affects v while mass is unchanged. If v is squared (as in the Ke calculation), then it will show the effect of the loss more than if it is not squared (as in momentum). Both are conserved consistently throughout the flight and impact of the arrow, but momentums are easier to compare. Ke is not because it is nearly impossible to calculate and thus compare the effects of friction and such.

Remember, all of these values are instantaneous. That way we keep the calculus out of it!

Hope that helps!

What kind of bow at what draw length? That is some almost unbelievable speed for 61# bow and 650 grn arrow.

Personally I would be happy with 160 fps with a 650 grn arrow. Should shoot through any elk.

That's a good point Shaun.It just goes to show you how little KE tells you about real terminal performance.Momentum is a much better indicator,all other things being equal.The momentum figure that Wingnut came up with for that setup is pretty high.Most compound setups wouldn't come close to that momentum figure but their KE would sound better.

If I had a bow that would move a 650 gr arrow,190 fps,I wouldn't care what the figure was.I can't imagine a more efficient setup.I'd like to know what bow that is also.I want one.

AS for the question about when is lighter more effective than heavier.  There is a point when lighter is more effective and it will be different for each bow.  If you doubt that go out in your yard with a golf ball, a baseball and a 16# bowling ball and throw them all overhand at a target 20yds away and see which one is the most effective.  I bet the bowling ball won't even get there.  I know this is an extreme example but the same principles apply.

There will be a definite bell curve if you sit around and try to figure it up, so there is a definite peak at which the heavier arrows will become less effective.  So for instance a 600 gr. arrow may preform the same at certain distances as an 1100 gr. arrow depending on tyour setup.