why do you float better in salt water
The basic reason that a human body floats better in salt water than in fresh water is that salt water is denser. The molecules of salt bond with water molecules, meaning that salt particles are suspended in the water. This gives salt water more matter per cubic inch than fresh water. Weight does not matter when it comes to buoyancy in water. Weight is simply a measure of the amount of force that is being exerted on mass. Mass is a measure of the amount of matter that an object or person contains. A person with a mass of 100 kg. on Earth will have the same mass on Mars, but the weight will be different because the gravity exerted on that mass is different. When you look at density, you are looking at the measurement of mass in a certain volume. Density is a measurement of mass compared to volume. For example, fresh water has a density of 1 gram per cubic centimeter. It comes into play because the less dense, physically speaking, a person is, the more they will float. Humans are largely made up of water, and their density reflects this -- the average man has a density of 0. 98 grams per cubic centimeter and the average woman is 0. 97 grams per cubic centimeter.
This is why most people can float in fresh water, like swimming pools. However, salt water has a density of 1. 024 grams per cubic centimeter. The reason salt water occurs in the first place is because of water's qualities as a solvent. Water molecules are polar -- one end of the molecule has a positive attraction and the other end has a negative attraction. Water molecules are also relatively small. This means that another polar molecule can be surrounded by water. The water gets in between the molecules and separates them from each other due to a very basic structure -- this is how things like salt are dissolved. This is also what makes salt water more dense. There are all these molecules of salt suspended in the water, and salt is more dense than water. If you've ever poured salt into a glass of water, you can see this for yourself -- salt sinks, straight away. When the salt and the water molecules come together, this changes the density of the water. Buoyancy has to do with how objects of different densities relate to each other.
A human mostly floats in fresh water because, on average, they are slightly more buoyant than fresh water. However, a human will sit lower in fresh water than salt water, because humans and fresh water are closer in densities than humans in salt water. If you took a stick of butter, which weighs about 4 oz. , and a piece of iron that also weighs 4 oz. and dropped them in a bathtub full of water, the butter would float while the iron would sink. You would notice that 4 oz. of iron appears significantly smaller than 4 oz. of butter, even though they weigh the same. This is because of density -- butter has 0. 86 grams per 1 cubic centimeter, where as iron has 7. 87 grams per cubic centimeter. Butter is less dense, and therefore more buoyant.
A floating object is displacing fluids that would otherwise fill the space it occupies. For example, a ball floating motionless on water is displacing the water and air that would normally be where the ball is. If we remove the ball, water and air will fill its space and soon everything will be motionless again. Just because that ball-shaped portion of water and air is motionless doesn t mean that it s weightless.
It does have a weight! But its weight is supported by the water and air that surround it. Because of the earth s gravity, the pressure of stationary water or air decreases steadily with altitude, so pressure exerted on the bottom of this ball-shaped portion is greater than the pressure exerted on its top. This unbalanced pressure produce a net upward force on the ball-shaped portion of water and air. That upward force is known as the buoyant force and it s evidently just strong enough to support the weight of the ball-shaped portion of water and air. If it weren t the ball-shaped portion would accelerate up or down. When we put the real ball back where it was and let it again float motionless on the water, the surrounding water and air continue to exert the same buoyant force on the real ball that they exerted on the ball-shaped portion of water and air. So the ball experiences an upward buoyant force that s equal in amount to the weight of the water and air it displaces. That observation is known as Archimedes principle.
Which brings me to your question. Here are two identical balls floating motionless on fresh water (left) and on salt water (right). In each case, the ball is experiencing a buoyant force that exactly cancels its weight. To obtain that exact buoyant force, the ball must displace a portion of water and air that weighs exactly as much as the ball weighs. Salt water is denser than fresh water, meaning that salt water has more mass per volume (more kilograms per liter) than fresh water. A liter of salt water consequently weighs more than a liter of fresh water. Displacing a liter of salt water therefore produces a stronger upward buoyant force than displacing a liter of fresh water. That s why the ball is floating higher on the container of salt water than it does on the container of fresh water. The ball doesn t need to displace as much salt water to obtain a buoyant force that supports its weight, so it rises higher on the salt water than it does on the fresh water. In each case, the ball finds just the right mix of water and air so that it displaces exactly its own weight in those two fluids.
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