Correct on gravity is gravity. A couple of ways you can look at it...and there may be no correct answer.
Don't be offended by my simplified comparison example as I have no idea how much "science" you have under you or anybody reading this.
Give me some assumptions we can make to simplify (unless one of these simplifications is the answer!). Ignore air resistance. You seal launch from a 10 m cliff with a 2m/s vs 5m/s Vh
. You land completely flat in calm water both times (which will take 1.428 s regardless of Vh
). You feel like the 5m/s has an easier landing right?
This, above, is probably the greatest simplification:
Seems the easiest answer is that the fluid reaction force (FRF) in the AP direction (which will be directed posterior) for the 5m/s condition will be greater. The fact that your upperbody is "independent" of the boat/lower body segment means as a result of that AP FRF you will incur flexion of the trunk.
Think about wearing only the lap belt in a car then rear-ending a car at 2m/s and 5m/s. You're upperbody is going to be thrown forward more quickly (towards the dashboard) during the 5m/s collision as your lower body essentially stays in the seat.
When you land from a waterfall drop with speed you will have some resultant velocity that increases only as Vh
increases as Vv
prior to impact will stay the same due to the constant 10m drop. That resultant velocity vector will have a FRF directed along the same line but opposite (Newton's 3rd) which will also increase only as Vh
stays the same regardless).
****The long technical answer****
Now what do you know to do when you are going to (unintentionally) land flat from a high drop...If you sit strait up the compression forces are directed upwards through the spinal column (compression mostly absorbed by the IV disks and a small % by the Vertebra) and shock in attenuated to the head in a vertical fashion. It is the high head acceleration (change from a head Vv
of 14 m/s to 0m/s) that results in the initial headache/neck pain you sometimes feel right after a drop. BUT we know to tuck! So the Fv
is absorbed this time by bending forces, compression on the Anterior side/tension on the Posterior side of the IV disks. This force is applied to the spinal column which is reflexively involuntarily countered by the back extensors due to feedback from muscle spindles. This results in a lesser head acceleration partially due to it moving through an arc (increases time so decreases force Imp/mom relationship) and not having the spinal column directly below it "forcing it to stop" in such a short distance.
So, what do you think? Sounds like a thesis or dissertation I'd act as an outside committee member on if you're a grad student! Strap a few 3D accelerometers on and seal launch!
It sure would have been nice to have this as a class discussion when I was an undergrad (though I never heard of kayaking then).