ORIGINAL: Paul L Mohr

It's not the same thing though because KE is only one part of equation. 60 ft/lbs is nothing like 60 lbs of sand, or gold. One is an energy figure the other is mass, two completely different things.

Kinetic energy is the work needed to acclerate a body of a given mass from rest to its current speed. So it's more of measurement of how something accelerates vrs how much energy it can transfer once it gets there. It has no direction, it is simply a measurement of how much work is being done.

Momentum has more to do with how an object slows down once it reaches a certain speed. It is directional in that it wants to continue in motion along the same line until something acts against it. Momentum is sort of the resistance to this force. Momentum is more simple as well as in if you double the mass or speed of an object you also double the momentum. The heavier an object is the harder it is to slow down.

So momentum is a much more imporatant figure when deciding penetration. A heavier object with more momentum also has the potentiel to transfer more energy to another object since it has more resistance to change and will want to continue in the same direction. It also retains speed for a longer distance. So a heavier object will hold more momentum over a distance than a lighter one will.

An extreme example would be which would you rather get hit with, a wiffle bat or a heavy wood one? I can swing a plastic wiffle bat much faster than I can a heavy wood one. Both probably producing the the same amount of energy when you compare rates of acceleration. However I bet the wooden bat is going to hurt a whole lot more because it won't want to slow down when it comes in contact with your body, or bounce off or deflect.

A more realistic comparison would be to compare an extremely light arrow to a fairly heavy one. Lets say a 200 grn arrow and a 800 grn arrow.

The 200 grn arrow at 370 fps would have 61 ft/lbs of energy and .328 lb/sec of momentum out of the bow.
At 40 yards the light arrow would retain 47 ft/lbs of energy and .290 lb/sec of momentum.
That is a 14 ft/lbs loss of Ke and .038 lb/sec of momentum.

Now the 800 grn arrow will produce 61 ft/lbs of Ke at 185 fps and .656 lb/sec of momentum right out of the bow.
That is twice the momentum at half the speed.
At 40 yards the heavy arrow will retain 58 ft/lbs of energy and .640 lb/sec of momentum.
For a 3 ft/lbs loss of Ke and .016 lb/sec of momentum.

So while shooting an ultra light arrow will work, it isn't the same as a heavier arrow even if they have the same Ke.

It is merely a measurement of how much energy your bow can produce, or how much work it can do.

The real question is always "How much do you need?". When is too little not enough and too much more than you need?

Paul