One of the most important things to take into
account is Newton's third law of physics:
"To every action there is always opposed an equal
reaction: or the mutual actions of two bodies upon each
other are always equal, and directed to contrart parts."
This means that when you exert a force on a surface,
it will exert a force back on you.
This can become very dangerous because parkour tends to
invole large drops that can easily injure or kill people.
For example, the human femur bone can generally withstand
about 4000 Newtons of force before cracking.
And according to Newton's second law, F = m a
so for an ideal man according to the hamwi method (48kg),
if this person accelerates at: 4000N/48kg = 83.33m/s^2
and then impacts a surface, their bone will probably
crack.
Now, since all humans are different, this number does
vary.
This number is also unrealistic though, because of how
impulses work.
An impulse is a force over a short period of time.
A good example is a car crash. In a crash, a car receives
a very large force in a small amount of time, which
increases the amount of damage.
The same goes for a large fall on the human body.
A landing roll can increase the amount of time this force
is spread out over, as well as move the force in the
horizontal direction to reduce the amount of force in the
impact.
The equation for an impulse is I = F/t
Roling lowers the force, which lowers I and increases the
t to also lower the I.