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.
Some big, heavy things — like the USS Enterprise — float in water. 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 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.
The answer is simply that the water below you is pushing up. There you are, underneath the surface in a swimming pool. When was the ball almost completely submerged or fully submerged but not quite sinking to the bottom? Do you get the same results with all the aluminum squares you test, or is there a lot of variation? Calculate the volume of the spheres for each diameter, using the fact that the volume of a sphere is equal to four thirds times pi 3.
Using the mass and the volumes, compute the average density of the aluminum sheet for each diameter by dividing mass by volume. At what density did the aluminum ball sink? At what density was the aluminum ball approximately equal to that of water? For each diameter of the sphere, what is the mass of the water that was displaced? For more accurate results, continue testing additional cm aluminum squares. Observations and results Did more and more of the ball end up below the top of the water as the ball's diameter decreased?
Was about half of the ball below the water when the ball had a diameter of about 2. If an object is floating in water, the amount of water that gets displaced weighs the same as the object. Consequently, while it was floating, the ball should have displaced the same amount of water as it decreased in diameter, and so the buoyant force should have remained the same.
However, the density of the ball was changing—it increased as the ball's diameter decreased. Density is the mass per unit volume—it describes how much "stuff" is packed into a volume of space. When the aluminum ball had a diameter of 6.
And as long as the ship displaces enough water to create a strong buoyant force, it can stay afloat—even if it is loaded with cargo. As the diameter decreased and density increased, the ball should have sank more and more.
When its diameter was about 1. This is when the ball had a density approximately equal to that of water. With a diameter of about 1. Already a subscriber? Sign in. Thanks for reading Scientific American. That means that a 1-foot-high column of water exerts 0. Similarly, a 1-meter-high column of water exerts 9, pascals Pa.
If you were to submerge a box with a pressure gauge attached as shown in this picture into water, then the pressure gauge would measure the pressure of the water at the submerged depth:. If you were to submerge the box 1 foot into the water, the gauge would read 0. What this means is that the bottom of the box has an upward force being applied to it by that pressure. This just happens to exactly equal the weight of the cubic foot or cubic meter of water that is displaced!
It is this upward water pressure pushing on the bottom of the boat that is causing the boat to float. Each square inch or square centimeter of the boat that is underwater has water pressure pushing it upward, and this combined pressure floats the boat. Sign up for our Newsletter! Mobile Newsletter banner close.
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