![]() Of course if the boat is heavier than the water it displaces, it just keeps falling. ![]() As more water gets pushed up, the falling boat and the rising water balance, so the boat floats. So with the water going up and the boat going down, is that overall stuff falling or rising? At first, as the boat just starts to go into the water, not much water gets pushed up, so there's net falling. As the boat sinks into the water, the displaced water (water that had been where the boat now is) has nowhere to go but up. The water that occupies as much space as the boat weighs more than the boat. That means that that density of the cargo, counting all that nearly empty space, is a lot lower than water. The key thing in any practical cargo boat is that there is a lot of air inside. There are several equally good ways to understand why a boat floats. I've marked it as a follow-up because we've partly addressed it before. Hey Rebbe Bob- Ellen told me to anticipate this question. Weight density means weight per volume, so this is the same as asking whether it is more or less dense than water. So whether an object floats in water depends on whether it weighs more or less than the same volume of water. If it's lighter, the pressure difference wins and it floats up to the surface. If it's heavier than the water it displaces, then the weight force wins and it will sink. The deeper water is at higher pressure, but that corresponds to only a small increase in its density.Īnyway, if you place an object in water that weighs the same amount as the water it displaces, the pressure and weight forces on it will just cancel, the same as they would for the water it displaced. In practice, water, like many other fluids, is only very slightly compressible. ![]() In other words, the deeper you go, the higher the pressure must get. That means that the water beneath it must be pushing up a little more than the water above it is pushing down. It would fall unless there were some force pushing it up, canceling the gravitational weight force. This means that if you have something that’s more dense than the water at the top but less dense than the water at the bottom, then it could "float" halfway in between - it’s more buoyant than the water at the bottom and less buoyant than the water at the top. So the water at the bottom is actually a tiny bit more dense than the water at the top. ![]() The water at the top doesn’t have anything above it, but the water at the very bottom has all the water above it pushing it down and squishing it together. But if it weighs less for how much space it takes up (like a cork), then it will float.īut water doesn’t always have the same density either. If something is more dense than water (like a heavy rock), then it will sink. So if you put something in water, all you have to do is think about how dense it is to figure out if it will sink or float. If something is more buoyant, then it is more likely to float to the top - i.e. This is basically what happens with buoyancy. If you mix something with a pretty high density (tightly packed) with something with a pretty low density (loosely packed), the stuff that’s more dense will settle to the bottom. Something’s density is basically how much it weighs for how much space it takes up (mass / volume) - you can think of this as how tightly packed it is. Whether you’re talking about water, air, or anything else, buoyancy happens because different things have different "densities". (Horizontal forces cancel.Buoyancy is basically a fancy word for how different things float. Their difference is the buoyant force \(F_B\). This pressure and associated upward force on the bottom of the cylinder are greater than the downward force on the top of the cylinder. \): Pressure due to the weight of a fluid increases with depth since \(P = h\rho g\).
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