# The Durability of Bone: Long Falls

1. May 21, 2013

### enterthetaiga

I am doing a paper on the physics in Valve's Portal and got interested in the "Long Fall Boots" that prevent any damage no matter how far you fall. I can calculate the force they can withstand and all of that, but I wonder how much more durable these boots are than plain human feet. So here is my question:

How much force is required to break a pair of human legs given that the person is in free fall and lands on her (the character in portal is an average-sized female) feet on a hard, sturdy surface (like tile), or how much energy can a pair of human legs absorb?

2. May 22, 2013

### Simon Bridge

The long-fall boot are total fantasy.

The maximum energy a bone can absorb without breaking is surprisingly small. The leg bones can absorb about 200 J - and you get two of them per human. (I'm pressed to find a reference though.)

Breaking a bone depends on the angle and type of the impact as well as the force of the impact.
Breakages from modest falls usually result from a bad angle of landing - for a very long fall, the knee joint will fail even if nothing else does.

You'll have noticed that if you land with bent knees it is less painful than landing with straight knees.

If you bend your legs then the momentum change when you land is spread over longer.

Also see:

3. May 22, 2013

### CWatters

Humans have a terminal velocity so above a certain height (call it Hmax) any increase in height makes no difference to how fast you land. So that limits how effective the boots need to be.

The maximium safe height without magic boots is only a few meters (call it Hs).

So one approach would be to say that the boots are Hmax2/Hs2 times as effective.

Bending the knees is recommended...

http://www.wikihow.com/Survive-a-Long-Fall