

"A lot of climbers don't really
understand the fall factor concept; however, it's pretty simple, even
if you hated math (this is math you can use in later life. In fact
, you can use it to have a later life)
Fall Factor is simply the length of the fall divided by the length
of the rope from faller to belayer. The equation looks like this;


Fall Factor = Length of Fall / Length
of Rope 

Fall Factor 2 is the maximum
you should encounter in a typical climbing fall, since the height
of a fall can't exceed two times the length of the rope. Normally,
a Fall Factor 2 can only occur when a leader who has placed no protection
falls past the belayer, or the anchor if it's a solo climb. As soon
as protection is placed, the distance of the fall as a function of
the rope length is lessened, and the Fall Factor drops below 2. 

Your life depends on the stretch of the rope... 
Shock load is
the result of three factors; The nature of the rope, the fall factor,
and the weight of the falling object. That is you.
Obviously, the only part of this equation that
can reduce the force of a fall is the bungeelike stretch of the
dynamic rope (unless, of course, you can lose weight really fast).
Thus, climbing safety systems are designed around the shockabsorbing
quality of dynamic rope. It cushions the fall, reducing the impact
force and the chance of system failure. In fact, the dynamic rope
is the one "given" in the whole system. It is designed
to limit the force of one climber's weight (80 KG) in a worstcase
fall (Fall Factor 2) to not more that 12 kN. Thus, the rest of the
gear can be designed to work with this known maximum force.
More rope means more stretch to absorb a fall.
Which explains why a Fall Factor 2 drop of 4 meters develops the
same shock force  9 kN  as one of 20 meters, assuming a dynamic
rope is used that conforms to UIAA standards. What's happening is
that the increasing length of the fall ( and the greater shock force
that goes with it) is compensated by the greater length of the rope
available to cushion its arrest.


Static rope doesn't stretch enough.... 
Static ropes  traditionally used mostly in caving
and rescue but now also used for sport rappelling and even in climbing
gyms  are designed to minimize stretch (cavers hate feeling like
yoyo's). So their ability to absorb shock is marginal, particularly
along short lengths of rope. What's more, static ropes aren't as
well defined by industry codes as dynamic ropes, so they vary in
elasticity according to the manufacturer and the country of origin.
They're often about as nondynamic as a cable, and transmit virtually
all the shock load to the safety system and the body. And in a climbing
situation, a very short fall can develop enough force to be critical.


Slings and runners are just like static rope...

Used for security, without a dynamic
rope, runners are just as dangerous as static rope. As the diagram
shows, a Fall Factor 2 develops enough shock load to risk failure
of the runner, the harness, carabiners, not to mention a lot of failure
in the climber's skeletal system. 

This is worth saying again: 
A fall of less than four feet on a
static rope or sling can create enough shock force to cause serious
injury or death. Bearing in mind that the human body can only handle,
for a brief instant , a shock force of 12 kN without risking serious
injury, you don't want to go around absorbing 18 kN. And you should
know that 18 kN is getting real close to, or over, the minimum limits
set by the UIAA on all the gear in your safety system. 

For purposes of comparison, here are the UIAA
limits; 
Anchors: 25 kN
Carabiners: 20 kN
Slings: 22 kN
Harnesses: 15 kN 

Meanwhile up at the 'biner.... 
Physics isn't
our friend in a fall. The same mechanical advantage we use in pulleys
works against us when we're on the end of a rope. Because at the point
where the rope returns, normally a carabiner, the force of the fall
is increased by approximately 66% (it would be doubled except for
the friction of the rope against the metal).
So, starting with our 9 kN maximum shock force
with a dynamic rope, the force on the carabiner becomes 15 kN in
a Fall Factor 1.9 fall. That's a lot. You better hope it's a good
anchor or placement .
Now apply that same math to a static rope,
The Factor 1.9 fall, with is normal shock force of 18 kN, becomes
a shock force of 30 kN (multiply 18 kN by 1.66) In this case, you
couldn't even count on a stout tree. And it wouldn't matter if the
anchor held, because something else would undoubtedly fail."


SOURCE from RockClimbing.com 

