Re: When bungee jumping, why is it as damaging to hit cement as water?

You've asked a good question, and I say that because I often judge a 
question by how easy it is to point to references, viz. books, that answer 
it.  I don't know of any books that deal specifically with your question!  
That doesn't mean there aren't any -- but I'm not familiar with them.

As soon as you come into contact with the ground, it starts pushing back.  
The problem (for you, having fallen) is that the ground is solid and you 
are not massive enough nor traveling fast enough to make it move out of the 
way so that you can pass into it.  Not will you compress it much.  The 
ground will completely decelerate you in a split-second by applying a very 
large force which, in the process, will break your bones.  On the other 
hand, if you were shaped like a a nail, your wedge-shaped tip would serve 
to push open a hole in the ground for you.

Now, water is a liquid, and can obviously move out of the way to let you 
in.  However, it is fairly dense, so that it won't move out of the way 
quickly.  So it will still exert a rather large force, resisting your push 
and decelerating you.

It's easy to move through air, not so easy to move through water, and very 
very difficult to move through ground.  Water is about 1000 times more 
dense than air, and ground can be 1000 times more dense than water.  
Density is the key factor.

I haven't talked much of surface tension of water, yet, because surface 
tension doesn't really play much of a role in this situation.  Surface 
tension is very weak.  It can allow an small insect to walk on water, but 
you'll simply fall through.

You can get a feel (pun intended) of the effect by simply slapping the 
surface of water, say in a swimmging pool or kitchen sink, with your open 
hand.  To convince yourself that surface tension isn't making a difference, 
you can "break" the surface tension with soap and try again.

One last note: if you try the above experiment, you'll notice that the 
speed of your hand when it contacts the water makes a big difference.  
That's why I said to slap it.  The drag force (skin friction due to fluid 
flow), whether it be due to water or air, is proportional to the square of 
speed.  Roughly speaking, it should hurt four times as much if you're twice 
as fast.

Now, I haven't really talked much about bungee-jumping, either, but I've 
tried to be rather general.  The fact that you are bungee-jumping when you 
hit the water doesn't change the physics of the impact any, it's your speed 
that makes the difference.  So if you design your bungee-jump right, you'll 
get to the water just as the cord has slowed you to a stop.

Finally, you might be interested in reading up a little on surface tension.  
Here are some links:

An article from a recent discussion on Usenet about 
surface tension and impacting water.  Click on "thread" to read more 
messages in that discussion.


a rather technical discussion of surface tension

Some experiments concerning surface tension

NASA's Quest site
from Sciences Explorer

Reeko's Science Site


surface tension for different liquids

Troy