Cannonball

[Ghost Post]

My simple calculations, assuming only bouyancy decelerating number 7, seem to indicate that he will come to rest (from 141m/s) only after travelling about 22m through water. Hence no. 7 should reach the bottom of a 10m pool.

[Ghost Post]

Make that 135m, not 22m.

[araya]

Clearly if the only thing slowing him down is buoyancy then he will reach the bottom of the pool, because the buoyant force is quite small. Of much greater importance is the amount of speed lost in the entry to the pool and the considerable drag force as he plunges through the water, which is proportional to the velocity.

[Ghost Post]

Agreed. Returning to my back-of-a-cigarette-pack calculations, to reach 10m depth, no.7 would require a velocity of about 38 m/s at the surface, assuming an instantaneous deceleration on entry into the water. This allows for a loss of about 4600 kJ. The question now is, would this amount of kinetic energy be dissipated on entry?

[Wonko the Sane]

Alright, I'm gonna have to dive in to some heavy physics tonight, but I'm determined to find this one out. I'm sure it's out there somewhere....

[hank]

My body is slightly buoyant ,and I remember cannonball dives wereby I went through a lot of pain, set up big splashes which resulted in considerable waves. The impact of a buoyant object on the water will result in an almost instantaneous deceleration transfering energy primarily to a resulting wave. Do we really have the tools to determine the loss of energy at impact? I know that naval architects often require models to verify there predictions with regard to efficiency of hull designs. Slight variations can result in considerable differences in the size of bow waves and other "surface effects".

For example, if you guys are using drag coefficients,( and if I remember correctly) the length of object has no bearing on the drag coefficient. However, it is well known in nautical circles that a longer boat can travel faster. Hmmm.

Okay, now that I've said my piece, go for it Wonko.

[kitakaze]

The thing about this cannonball problem is that the cannonballers are spheres.

My son has some slightly bouyant eggs (toys).

When I drop them into his bath from a height of 1 metre, they don't make a big splash because of the shape (unlike a human cannonballer, which has an irregular shape).

And they easily hit the bottom of the tub.

Kaze

[Wonko the Sane]

The problem with that is that a 1 meter sphere is a lot larger than an egg. Try something larger and it may not work. I've given up on this since the actual drag induced on entering the pool is chaotic (it changes as waves are produced, almost randomly). Even though they're considered to be perfect pools, a 1 m sphere has a lot of room to get effected. I fold, anyone else willing to give it a shot?

[araya]

I would tend to agree with you, kaze. I also agree with you Wonko.. it's not worth the difficulty to calculate. I *could* run to my fluid mechanics textbook and start calculating flow coefficients, reynolds numbers, and trying out turbulent flow equations but it's the sort of thing you'd never know for sure until you tried it. Any volunteers? I know this guy with a swimming pool..

[Wonko the Sane]

Realistically, we can't try this for two reasons. 1) getting a sphere of the right density and making it "perfect" is hard. 2) Very few pools are almost 35 feet deep. 3) You'd need a helicopter and some really still air to get an accurate enough drop. But anyone that tries it I will travel to wherever you are and worship at your feet for having that kind of balls, as you'd have to be almost more nuts than me.