Skill 8 of 9
In Progress

Falling & pain

It’s not the fall that kills you; it’s the sudden stop at the end

Douglas Adams[1]

Obviously, just carrying momentum is no problem; a free falling skydiver has a good time in the air. But quickly changing that momentum is an issue; landing without having opened the parachute will be something he can feel…

What exactly causes his pain and damage at impact? Let’s look it through, one step at a time:

  • Apparently, momentum $\vec p$ in itself doesn’t “feel” like anything; a change in momentum $\Delta \vec p$ is needed before we feel anything.
  • A change in momentum $\Delta \vec p$ means a change in velocity $\Delta \vec v$.
  • And that requires acceleration $\vec a$.

So, is it acceleration $\vec a$ we feel?

No, still not. An astronaut in a space shuttle in orbit around Earth is in constant radial acceleration in his circular motion,1 but inside that space shuttle he feels weightless. He does not feel the acceleration that is happening constantly.

But try to pull someones arm. This can be felt. You are causing an acceleration in this arm which is different from the acceleration of the rest of the person’s body. That makes the shoulder joint strain a bit – and that is the key! That is what we feel! Not acceleration $\vec a$ itself, but differences in acceleration $\Delta \vec a$ between different parts of our body.

  • When the skydiver falls to the ground and lands feet first, the ground applies a force $\vec F$ on his feet to
  • cause an acceleration $\vec a$ that stops the feet’s motion (reduces their momentum to zero $\vec p_\text{feet}=0$).
  • The feet are slowed down in a split second, but the rest of the body isn’t. With the rest of the body still having downwards momentum $\vec p_\text{body}$, it is now his ankles and the bones in his legs that have to apply a force large enough to cause the needed acceleration to slow the rest of the body down over a split-second.

If the bones, joints, tissue, organs and other organic parts of the human body can’t create the forces needed to slow down the rest of the body, then they break, tear or in other ways are damaged. Such damage is by our brains interpreted as pain.

In summary: pain is the human conception/feeling of differences in acceleration $\Delta \vec a_\text{body parts}$ between body parts. That is what (mechanical) pain is.2

You do not need a parachute to skydive. You only need a parachute to skydive twice.



  1. Douglas Adams Quotes’ (web page),, STANDS4 LLC, 2020, (accessed Apr. 10th, 2020)