This equation accounts for the equivalent bulk modulus, which describes the compressibility of the fluid. The sudden impact of fluids can be quite forceful. See also Why is jumping into water from high altitude fatal? Water molecules have hydrogen bonds in between them that hold them together, which means that they are not easily displaced.
So when something collides with water or tries to pass through it, the water molecules resist being displaced and this creates a reaction force against the object. This is also what causes drag in general. The attraction force between the molecules determine its viscosity, and this in turn determines the resistance to displacement and therefore the drag.
And water is much more viscous than air, so bullets slow down much more in water. As to your second question about why some bullets travel faster than others in water, its because the drag depends on the shape of the bullet.
If a bullet has a smaller cross-sectional area and surface area, and is more 'pointy' towards the head, it will be more penetrative in water. Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. Why water is so good at stopping certain bullets? Ask Question. Asked 4 years, 2 months ago.
Water molecules are packed much more tightly together than air molecules and therefore in water, there are many more collisions as the bullet moves forward, and the bullet stops much more quickly. While the episode, which also features Olympic medallist and mathematician Cameron McEvoy, demonstrates the effect that different fluids have on resistance, Professor Taylor says we also have to take into account other properties, like the shape of the moving object, its speed, and the amount of energy it carries.
The shape of a bullet is designed so that, as it moves through the air, it collides with as few air particles as possible, and each collision transfers the least amount of energy. Picture a billiard ball glancing off another ball and then compare that to hitting one head on. The glancing ball will retain most of its energy and speed, while the head on collision may stop the ball dead.
A bullet-like shape creates more glancing collisions and fewer head-on collisions than a shape that is flat in the direction of travel. So it travels much further before it loses its energy. Ellen Ioanes. They work by creating a gas bubble around the tip of the bullet, reducing drag when a bullet is shot through water. Typical bullets can travel just a few feet through the water before they're slowed to a stop. CAV-X bullets can reportedly travel 60 meters underwater, and can go through 2 centimeters of steel fired from 17 meters away, indicating that it could even be used to penetrate submarines.
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