Some big, heavy things - like the USS Enterprise - float in water. Some small, light things, like a ball bearing, don't. So size doesn't determine whether...
When I jump into a swimming pool, I sink like a stone unless I flail about wildly, but when the USS Enterprise aircraft carrier – which is just a tad bigger and tad heavier than I am – jumps into the ocean, it floats with no effort whatsoever. What’s going on? Let’s wonder a bit about the science of floating.
Some big, heavy things – like the USS Enterprise – float in water. Some small, light things, like a ball bearing, don’t. So size doesn’t determine whether something floats, and neither does weight. Let’s start by identifying what causes something to float or not.
Everything on Earth, whether on the surface or under water, has the weight of everything higher up pushing down on it.
Everything on Earth, whether on the surface or under water, has the weight of everything higher up pushing down on it. Yes, right now as you read this, the weight of all of the air above you, right up to the top of the atmosphere, is pushing down and in on you. The resulting force is about 14.7 pounds on every square inch of your body. 14.7 pounds per square inch, or psi, is the normal, average atmospheric pressure on the Earth.
If you dive down into water, you’ll not only have the weight of the air up above pushing in on you, but the weight of all of the water above you as well. Water is relatively heavy – go down just 30 feet and the weight of the water per square inch on your body will equal the weight of the hundreds of miles of air above you.
Why doesn’t the weight of all that air and water push you down, down, down, after you dive into a swimming pool? The answer is simply that the water below you is pushing up.
Water wants to be where you are
There you are, underneath the surface in a swimming pool. Water wants to be where you are – your body has displaced a whole lot of it. If you suddenly disappeared, water would rush in to fill the space. It is the force of that potential “rushing in” that results in a force acting against the weight of the air and water above you. This is the buoyant force.
The buoyant force depends on how much water an object displaces. The larger the object, the greater the buoyant force it experiences. Ah, but will that object float?
An object floats when the buoyant force is large enough to counter the object’s weight. So a large hollow object might float because large means more water displaced – so more buoyant force – and hollow means relatively little weight. A small solid object might not float, however. Less water displaced results in a smaller buoyant force. If that buoyant force isn’t enough to counter the weight of the object, it will sink.
So yes, the USS Enterprise weighs far more than I do. But the weight of the water it displaces is more than the weight of the aircraft carrier, so it floats. Me, I weigh more than the water I displace, so I sink.
You can get a feel for how much water the USS Enterprise displaces from this photo:
The keel of the boat is deep under the water, and the ship is quite wide for most of its length. So that’s a lot of boat volume under the surface, all of which is displacing water.
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Questions to ponder:
If you roll a blob of clay or putty into a ball and drop it into a pot filled with water, it sinks. But it floats after you flatten it and curl it up into a cup shape. (Try it!) What’s going on?
A person can float effortlessly in the Great Salt Lake or the Dead Sea. Why is that possible?
FAQs
If that buoyant force isn't enough to counter the weight of the object, it will sink. So yes, the USS Enterprise weighs far more than I do. But the weight of the water it displaces is more than the weight of the aircraft carrier, so it floats. Me, I weigh more than the water I displace, so I sink.
Why does a ship float but humans sink in water? ›
The amount of buoyancy force that an object experiences is equal to the weight of the fluid that it displaces. The average density of a ship is much less than the average density of a human being.
Why does a ship float on water and needle sink? ›
The iron ship is constructed in such a way that it is mostly hollow from within Hence the average density of the ship is less than that of water (i.e. less than 1 g/cm3 ) wheres iron needle being compact and its density is 7.6g/cm3 which is more than 1 g/cm3 (density of water) Hence the needle sinks in water.
What is the science behind ship floating? ›
This force is called buoyant force. The buoyant force pushes upwards against the object. Gravity exerts a downward force on the object (its weight), which is determined by the object's mass. So if the force exerted downward on the object by gravity is less than the buoyant force, the object will float.
Why does a nail sink but a ship float? ›
The density of nail (as of iron) is much larger than the water. So it sinks easily. The weight of the water displaced by the ship is equal to its weight, so it floats. Whereas the weight of the waterdisplaced by the iron nail is less than its weight so the iron nail sinks.
Why do you have to swim away from a sinking ship? ›
Swim away from the boat to avoid the propeller. Although some have busted the whole "ship pulling you under" myth, actual survivors have reported it happening.
Why did the Titanic sink buoyancy? ›
Whether or not an object floats has more to do with density than weight. For example, ships like Titanic weigh tons, but typically float along just fine because they are designed to be less dense than the water around them. The reason Titanic sank is because the density changed when the ship hit the iceberg.
How does a cruise ship float and not sink? ›
Colossal vessels stay above water by displacing an amount of water equal to their mass (the wide, U-shaped hull helps with this). As the ship moves forward and pushes water away, the water is ceaselessly trying to return to fill the space, with an energy that forces the ship upward.
Are ships made of iron? ›
A ship is a hollow object made of iron and steel which contains a lot of air in it. Air has a very low density. Due to the presence of a lot of air in it, the average density of the ship becomes less than the density of water, therefore, a ship floats in water.
Why don't ships sink but a pin does? ›
Density of pin is more than water so it sinks. The density of an iron ship is mass of the ship divided by the volume of the ship. Although the ship is made of iron, the volume of the ship is so huge that density becomes less than the density of water. The reason being, the ship is not a mass of solid iron.
Ships float on water because of a principle called buoyancy. Buoyancy is the upward force that a fluid (such as water) exerts on an object that is submerged in it. The amount of buoyancy force that an object experiences is equal to the weight of the fluid that it displaces.
What is the floating ship theory? ›
Archimedes' Principle
If an object is completely submerged, this buoyant force, pushing upwards, effectively reduces its weight: it seems to weigh less when it's underwater than it does if it were on dry land.
Why do ships float but people sink? ›
An object will float if it weighs less than the amount of water it displaces. So here the weight of the ship is less than that the amount of water it displaces. Hence it floats on water.
Why do ships sink if wood floats? ›
It depends on both. Whether or not a ship sinks or floats is determined by both its total weight and the weight of water it displaces. If the former is bigger it sinks, if the latter is bigger it floats.
Does a coin float on water? ›
The reason the pennies sink in water is because of an idea called density. The pennies have more density than the water, and so the pennies sink. Anything with more density than water will sink in water, but other objects that have less density than water will float.
Why do humans float and not sink? ›
Fat has a specific gravity of less than 1.0 and floats in water, while both bone and muscle have a specific gravity of slightly more than 1.0. Thus, persons with a high proportion of fat will float while some individuals with very low fat levels, heavy bones, and high muscle mass will sink.
Why does the human body sink in water? ›
Bodies whose densities are more than that of water sink in water. The density of the human body is less than that of the water. Therefore, when a human dead body falls in water, it floats for a few seconds in it. But when water goes into the body, its density becomes more and it sinks.
Why a ship can float on water but a pebble will sink? ›
Ships float because they have empty spaces at the bottom and experience large buoyant force from the water below it. In contrast to this, the stone is denser than water, does not have empty spaces and does not experience enough buoyant force from the water below it, so it sinks.
Why is it easier for a human to float in the dead sea than in a freshwater lake? ›
The water of the Dead Sea is full of salt, which makes it much denser and heavier than freshwater. If you swim in it, you float very easily.
