What affects Buoyant force?

Question

Why isn't buoyancy affected by the object immersed in water? I know that more depth means more pressure means more buoyancy. But based on Newton 3rd law, the water should apply a force back on the object immersed so no matter the weight of the object, it will always float. BUT I KNOW THIS IS FALSE, I just dont know why.

Answer 1

I know that more depth means more pressure means more buoyancy.

Your intuition got you onto the wrong track. But you are partially right: More depth means more pressure. More precisely: Pressure is proportional to depth.

Look at the following picture showing two equal bodies at different depths.

At greater depths the pressure forces (shown as red arrows) increase. But you also need to consider the direction of the forces. The forces everywhere point into the body (perpendicular to the surface of the body).

Therefore, with increasing depth not only the force acting on the bottom side of the body (pointing upwards) grows, but also the force acting on the top side of the body (pointing downwards) grows. That's why these forces partially cancel each other.

In the end the vector sum of all these forces turns out to be independent of depth. It depends only on the volume of the body. The buoyant force is equal to weight of the displaced water. This is Archimedes' principle. See also question "Is there a reason behind why Archimedes principle works?" and its answers.

Answer 2

Greater depth does mean greater pressure. However a submerged incompressible object's buoyancy does not change. The pressure on the bottom of the object will be higher than the pressure on the top of the object because of the differences in depth. The difference in these pressures is equal to the weight of the volume of liquid it displaces. If the object weighs more than the volume of liquid it displaces, the difference in the top and bottom pressures, it will sink.