It's in the physics....
The table saw blade is a flywheel that stores a tremendous amount of energy.
= is the angular speed
I = is the moment of inertia
K = the kinetic energy
Not only does the blade store a bunch of energy, but the motor on a good saw is another flywheel with 3 to 10 times as much inertia. This is why good saws have large motors and they will keep a steady speed as the load changes.
When we cut on the table saw, we are sticking a piece of wood into this powder keg of energy. If everything works right, the blade slices through the piece and we get a nice cut. If something goes wrong and the blade catches the wood, the energy is all transferred into the wood. This is why a kickback is so dangerous.
Some calculations on a typical saw:
Blade RPM = 4400
Blade weigh = 28 ounces
Blade Diameter =10 in
I= 0.0064 Kg mA2
K= 680 Joules
3 hp motor
Lets be safe and rip a 4 foot long 2x4:
48” long 2x4
m= 2 kg
v= 42 m/s = 90 mph.
If we experience a kick back that momentarily stops the blade and motor:
That 2x4 is going 90mph!!!! OUCH.
Even though we made a lot of assumptions to come up with these numbers, it explains the stories of being hit with a piece of wood which “continued on and across the shop and broke the window on the way out”.
It is not likely that all of the motor and blade energy is going to be transferred into kickback velocity, but a significant portion might be. So lets revise our assumptions:
Destructive efficiency = 50% ( how much of the energy is coupled out)
The 4’ 2x4 is still going 45 mph. I don’t know too many people who have accidents with 4’ pieces, it is the shorter smaller pieces that cause problems. A 2’ piece would be back up to 90 mph. ( 2’ is the national average reported in the 2001 consumer safety survey)
What does this mean?
This is only a theoretical discussion and the coupling of the blade’s flywheel energy into a kickback is highly variable. The important point is that a table saw has a lot of energy stored in such a way that it can be destructively directed towards the operator.
It is interesting to note that it takes about 88 joules to break a human arm. That is only 5% of the stored energy in a typical table saw’s blade and motor.