# How things Float

• B
Gold Member

## Main Question or Discussion Point

Imagine a container containing water up to 100cm of its height then I dip a cube of plastic in the water on depth of 75cm. We all know it will float because of its less density, but if we go with Pascal Law, The pressure/force applied by the water above the cube is more than than the buoyant force/pressure applied by the water below, and if the buoyant force is less then the cube should not float. But when did the same at my home in a bucket the cube came to surface. Why??

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stockzahn
Homework Helper
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Imagine a container containing water up to 100cm of its height then I dip a cube of plastic in the water on depth of 75cm. We all know it will float because of its less density, but if we go with Pascal Law, The pressure/force applied by the water above the cube is more than than the buoyant force/pressure applied by the water below, and if the buoyant force is less then the cube should not float. But when did the same at my home in a bucket the cube came to surface. Why??
Pressure increases with the depth of the water, hence the pressure at the bottom of the cube is higher than at its top. If the difference of these two pressure forces is larger than the weight of the cube, it floats. You must not compare the "height" of the water below and above the cube, but the pressure vs. the depth.

Dale
Mentor
The pressure/force applied by the water above the cube is more than than the buoyant force/pressure applied by the water below,
This is incorrect. I am not sure where you got this idea, but it is exactly backwards. The pressure on the top is less than the pressure on the bottom. This is the cause of the buoyant force

Gold Member
The pressure on the top is less than the pressure on the bottom. This is the cause of the buoyant force
Well, I know that sir but if their is more water above than below then, the water above push the box down. Right??

Gold Member
Pressure increases with the depth of the water, hence the pressure at the bottom of the cube is higher than at its top. If the difference of these two pressure forces is larger than the weight of the cube, it floats. You must not compare the "height" of the water below and above the cube, but the pressure vs. the depth.
If their is more water above than below then, the water above push the box down as it will apply more downward force than the upward force . Right??

stockzahn
Homework Helper
Gold Member
If their is more water above than below then, the water above push the box down as it will apply more downward force than the upward force . Right??
Only if the water above and below the cube is not "communicating" (e.g. in two separated compartments). The weight of the water above the cube acts around the cube on the water below the cube and increases the pressure. And why shouldn't it? Otherwise that would mean that the weight force of all the water above the cube is lying on the top of the cube.

Dale
Mentor
Well, I know that sir but if their is more water above than below then, the water above push the box down. Right??
No. The amount of water below is completely irrelevant.

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Only if the water above and below the cube is not "communicating" (e.g. in two separated compartments). The weight of the water above the cube acts around the cube on the water below the cube and increases the pressure. And why shouldn't it? Otherwise that would mean that the weight force of all the water above the cube is lying on the top of the cube.
Sorry but i didn't understand

Dale
Mentor
@Pranav all that matters is the pressure. The pressure is determined by the depth of water above. The amount of water below does not matter. The pressure increases as you go down, therefore the bottom is always subjected to a greater pressure than the top.

Gold Member
@Pranav all that matters is the pressure. The pressure is determined by the depth of water above. The amount of water below does not matter. The pressure increases as you go down, therefore the bottom is always subjected to a greater pressure than the top.
And more the pressure more you will be pushed down??

stockzahn
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Gold Member
Sorry but i didn't understand

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Dale
Mentor
And more the pressure more you will be pushed down??
No, you are pushed in a direction normal to the surface. That is down on the top, up on the bottom, and inwards on the sides.

So the upwards force on the bottom is greater than the downwards force on the top. The net force is therefore upwards (buoyant)

Last edited:
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Thanks you everyone.

• russ_watters
Gold Member
Now it's clear to me.