# Homework Help: Why bellyflops hurt?

1. Oct 17, 2006

### blaster0

If a body crashes into a water surface at high speed, the impact is almost as hard as on a solid surface. Explain why.

I think it is something like this:
The water cannot displace fast enough and so all the molecules are close together as if they were a solid.

Also:
The Earth rotates at over 1000km/h so why when you jump off should the wall not slam into you?

I think it is that the entire Earth (even until the edges of its magnetic field rotate at that speed, and so it does not matter if you are on the ground or in the air because the entire system is moving.

2. Oct 18, 2006

### blaster0

All help is appreciated, i have this due soon

3. Oct 18, 2006

### OlderDan

You could give a reason why it is harder for water to get out of the way than the air you were falling through. Compare the density of water to the density of gas and the density of a solid.

You must have left some words out of the second question. You are on the right track. What is your motion relative to an observer out in space before you jump?

4. Oct 18, 2006

### andrevdh

I also have heard this expression but never have seen an explanation for it. I tend to agree with your first guess at it: the inertia (getting quickly out of the way) and basically incompressibility of water. Another factor to consider is the surface tension (keeping it together) of the water.

I would think that in order to survive such a process one should hold your arms straight along the sides of your head with your palms pressed together. This should break the surface tension as the tip of your fingers enter first and allow you to slip into the water rather than slam into the surface which resists breaking and elastically deforming under the impact. Once in the water you should bundle up in order to slow down as quickly as possible before hitting the bottom.

Last edited: Oct 18, 2006