Do you remember the first time you went swimming? That must have been a very exciting experience, wasn’t it? If you think about it, though, swimming is not really something you should feel excited about. Looking into some of the horror stories associated with it—all those drowning and sinking—you’ll find it surprising why people love to swim.

But why exactly do you sink in water? It doesn’t matter how good a swimmer you are. Once you stop moving arms and feet while in the water, you’ll start sinking. How is that possible?

It’s Not Your Weight

You may think that it’s because you are heavy. Well, that’s one possible reason, considering that there are a bunch of objects smaller and lighter than you that float on water. But surely there are a lot of them that sink as well. For instance, if you drop a coin or a needle in the water, it will sink in split second. To add to the confusion, sea vessels as heavy as 400,000 tons conveniently drift on the sea. So what is it that affects an object’s ability to float on water?

  • Density
  • Everything has density—your car, shoes, food, and even the device you’re reading this from. The very air you breathe has certain density. This property of matter is defined as the mass of an object per unit volume, or simply how much a certain volume of an object weighs. The smaller an object’s volume is compared to its mass, the higher its density.

    Under the microscope, you can determine density by observing the distance between atoms or molecules. The closer they are to each other, the higher the density of the substance they form.

    Water has density of its own, but it’s quite lower than that of most substances. When an object’s density is lower than the density of the section of water where it is placed and given enough upward force caused by the pressure difference in water, it will float.

  • Buoyancy
  • Pressure increases as you go deeper into the water. That’s because the weight of the overlying fluid pushes down the weight of the underlying fluid. This pressure difference creates an upward force that is equal to the downward force created by the object placed on it, causing that object to float. This principle is called buoyancy, and that’s what makes ships float on water. Given the fact that the ocean contains other substances, primarily sodium chloride, its density is higher than that of fresh water, making it even easier for ships to float.

Density and buoyancy are crucial factors to consider when designing and building not just sea vessels, but also personal flotation devices (PFD). With the right combination of properties, you can create the most effective sink-resistant objects needed in the maritime industry.

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