[Physics] Why does the block on the left still experience buoyant force in this image?



Hey guys, so my question basically boils down to, in the linked image below, **why is buoyant force exerted by the fluid** ***surrounding*** **the block instead of just the fluid** ***underneath*** **the block**, when buoyant force direction is straight upwards only? Like what property of the fluid molecules on the sides and top of the block is causing them to pull the block upwards?

Or in other words, there’s no water below the block, so what is pushing it upwards apart from the normal force? Thanks.


In: Physics

Well, the buoyant force is a property of the liquid, not the object. The force is gonna be present, regardless, but due to the density and mass of the object, that force is not enough to overcome gravity in that case. It’s similar to friction in that regard, hence why you see the normal force also being exerted there.

EDIT: Oh yeah, another way to think of the buoyant force is as a product of the density of the fluid. A denser fluid will have more molecules to “push” against any objects that are submerged in it and vice versa for a fluid with less density.

The buoyant force is always present in water, the theory is that there isn’t enough buoyant force to keep the block floating, or in other words, the weight of the block is greater than the buoyant force, so the block sinks.

Edit: I didn’t really read the question but yes, there is water pressure, or buoyant force in each direction, however the ones on the sides cancel each other out, as there are no additional forces in the horizontal directions. This is not the same for the vertical direction, where there is the additional force of the weight.

If you imagine that the block is wrung to the bottom of the water container similar to gauge blocks and there is no water between the surfaces, or any avenue for it to enter… then I don’t think there would be a buoyant force.

This is because buoyant force is a pressure difference exerted by a fluid and if there isn’t a fluid under the block it can’t exert said force from that direction. Of course in any normal situation there would be some tiny amount of water intrusion under the block and the fluid on the sides would be responsible for applying this difference in force. This is because if we imagine a column of water with a cross-section of our block going straight up compared to a similar column of pure water, if our block is less dense than an equivalent volume of water then the pure water column will have more weight and pressure. Higher pressure under the pure water column compared to lower pressure on the column with the block would result in water pushing under it, forcing the block upwards.

But in this case without any water under the block and no avenue for it to enter without the block rising from the bottom surface, I don’t see any way for there to be any water pressure under the block at all, much less the pressure difference which is the buoyant force. Of course this isn’t what the exercise is looking for so don’t put that on a test or anything.

Think of the water being divided into really thin horizontal strips.

The topmost strip of water is just kinda… there. It wants to spread out across its container. And if the block encounters it, the water will squeeze the block a little bit as it wants to spread.

The next strip below does the same thing, except now it has the top strip of water on top of it, weighing down on it. The weight of the water above makes it squeeze on the block just a little bit harder.

Each subsequent strip down increases this squeezing effect due to the weight of water above it. This is why water pressure increases only as a function of depth, no matter how much water you actually have. A big, deep pool will have the exact same pressure at the bottom as a tall, narrow column of the same height.

Buoyancy arises (ha) because when you put any 3D object into water (or any fluid, for that matter) the part of the object that is deepest submerged is being squeezed slightly harder than it is at the top. A higher pressure at the bottom an a lower pressure at the top results in a net force that causes the object to feel an upward force. If that force is greater than the object’s weight, it will float upwards until this buoyant effect balances its weight force exactly.