How do people do shallow dives from so high up, without dying?
In: Physics
Newton worked out that for anything going relatively fast into a liquid (or really fast into a solid, like a bullet hitting a tree, because at those speeds solids behave like liquids) the “impact depth” (the distance into the liquid the fast object travels) is made up by the ratio of the object’s density to the liquid’s density multiplied by the object’s length. Speed makes no difference at all. The faster you hit the water (or anything hits anything) the more force you have, but equally the more force the water being displaced has to push back against you, and so it cancels out, resulting in the impact depth equation.
A human in diving position (so hands above head to maximise length) is about 2m long. Humans are mostly water so our density is about the same as water and so our density ratio with water is about 1. This means that no matter how high up a person dives from they are only ever going to travel 2m into the water, and so any depth of water beyond 2m doesn’t serve any purpose. 2m is always enough.
So then the thing about diving from high up is its all about whether you can survive the rapid deceleration from the speed you hit the water at (which for cliff divers can be up to 90kph) over only two meters. Surviving rapid deceleration is about making sure the effects of deceleration are spread evenly over your body (so correct position when entering the water), and about tolerance for pain and having strong bones that don’t break under severe pressure, but above a certain point the deceleration just isn’t survivable. There’s a lot of debate about what exactly the maximum safe height is, but anything above 100 feet is pushing it.
Often cliff divers will jump into broken or aerated water since that reduces the density of the water changing the ratio and so allowing them to dive deeper.
They disrupt the surface tension of the water.
Here is a clip from Guiness World Records of the highest shallow dive and explaining how he’s okay. There’s a slow motion clip where he brings his hands together and hold them below where his body is, so his hand hit the water and disrupt the tension so the rest of his body can enter the pool without hurting himself too badly. He also spreads himself out to help him decelerate and pushes forward slightly to displace his downward energy.
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