Let us assume that the an object collides into me in one of two scenarios, either I’m standing on shore, or else we’re submerged underwater. The object doesn’t change it shape, and possesses the same velocity prior to the collision. Would the consequences be the same? more severe? less severe?

Points to take into account:

  1. The "wet" weight is larger than "dry" weight, thus the hitting body has more momentum prior to collision. This is true especially if for instance the hitting body has some cavity which may be filled with water thus giving it larger effective mass during collision.

  2. My another concern is the effect of the medium. Since water are incompressible, I should start "feeling" the body "pushing" water towards me earlier (prior to impact), thus the effective duration of the collision is longer, which should imply smaller forces.

  3. Aside from that, since the collision in water cannot be thought of as sudden, how should I acount for drag forces during the process.

  • $\begingroup$ You appear to be confusing weight, mass, and momentum. $\endgroup$ Jul 19 '18 at 17:16
  • $\begingroup$ I don’t. I well aware of differences. I might have mistranslated to English, but when a body with cavities is submerged, the cavities fill with fluid. As a result, its engines now propel both the mass of the body itself, plus the enclosed fluid. Thus I believe that its effective mass and therefore momentum prior to impact is enhanced. $\endgroup$
    – Yair M
    Jul 19 '18 at 17:21

Let us not consider the wet weight, because we don't at this stage have enough info on the size and location of cavities and how fast the can shed the water.

As for the duration of the impact it can be very short, crashing impact. Consider an sledge hammer hitting a glass panel under the water, assuming the same impact velocity, it will impart same force. I don't think there will be cushion of pressure wave in front of the hammer to break the blow, not significantly, water is incompressible.

An obvious difference is the way our body responds. On the air our body will roll with punch and take the blow in a more flexible way. If you hit me with a fast ball in my torso my rib cage will bend in under the impact and my longs will dampen the blow, may be by bulging out around and acting like shock absorber, but if they are pre-pressured with the hydrostatic pressure they will resist the punch in a less flexible way and chances of tissue injury are higher.

If we consider the viscosity of water added as a shear friction to the entire surface of our body, it will be that much harder to recoil, hence it will receive the impact more forcefully. Although if wear some wetsuit with lubricated skin it may help.

All these, of course if we assume we somehow manage to move the ball or projectile with the same speed below the water as above.

** EDIT **

After OP's adding more detail:

Something that is semi autonomic and follows you has to have fault safety algorithm built into it. Such as it will default to a fast left turn and stop, or suddenly dives down and slows in a circle ready to be towed back. You may be able to attach a soft cone to the tip as a sacrifice soft structure to break the impact, etc., etc.

We my google about design of torpedoes and submarines.

  • $\begingroup$ Thanks for the detailed response .The system I consider is a toy submarine. It has engines, thus I can control its velocity. I can also estimate the volume of water enclosed within the hull (parts of it aren't dry). I'm concerned by damages to the sub, and injuries to a person upon impact. Does these details give more insight? $\endgroup$
    – Yair M
    Jul 19 '18 at 7:35
  • $\begingroup$ @YairM how about you pilot it more responsibly in the first place,then? $\endgroup$ Jul 19 '18 at 17:17
  • $\begingroup$ I want it to follow me. That’s its goal. I’m concerned what happens if during test runs it doesn’t stop fast enough. $\endgroup$
    – Yair M
    Jul 19 '18 at 17:24

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