Why would an object become lighter under water?

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In summary: The object's weight will not change as it is lowered into the water. In summary, when an object is suspended beneath the surface of a lake, its weight is offset by the weight of the water it displaces, resulting in a decrease in its apparent weight. This is due to the pressure acting in all directions under water. However, the object's weight will not change as it is lowered deeper into the water.
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Kaxa2000
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If you suspend a large object beneath surface of a lake...it seems to weigh less than if it was above...why is this? Would it become even lighter if you kept lowering it into the water?
 
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Kaxa2000 said:
If you suspend a large object beneath surface of a lake...it seems to weigh less than if it was above...why is this? Would it become even lighter if you kept lowering it into the water?
The object's weight is offset by the weight of water it displaces. If the object is 1 cubic metre, it will displace 1000kg of water. That 1000kg of water will try to push its way back into the space occupied by the object. In doing so, it will push up on the object with a force of 1000kg.

An object of sufficient rigidity will not get lighter if it is pushged deeper below the surface since it will displace no more than 1000kg of water at any depth.


Unless this is homework.

In which case: what are your thoughts?
 
  • #3


Kaxa2000 said:
..why is this?
Why is a very good question.
The simple answer is that there is an upward force equal to the weight of the water displaced - but that doesn't really explain why!
The important thing to understand is that under the water the pressure acts in all directions.
If you had a cube of water under the surface there is a force upward exactly equal to the weight of the cube of water.
The water doesn't move so the force up must be equal to the weight of this cube.
No remove the cube and replace it with another cube of another material.
The force upward doesn't change - so the new cube has an upthrust equal to the weight of the cube of water

Would it become even lighter if you kept lowering it into the water?
No - assuming the density of the water doesn't change
 

1. Why does an object appear lighter when submerged in water?

When an object is submerged in water, it displaces an amount of water equal to its own volume. This creates an upward force, known as buoyancy, which acts against the object's weight. This results in a net upward force, causing the object to feel lighter.

2. Does an object's weight change when it is submerged in water?

An object's weight, or the force of gravity acting on it, does not change when submerged in water. However, the buoyant force acting on the object counteracts its weight, making it feel lighter.

3. Does the density of an object affect its buoyancy in water?

Yes, the density of an object plays a significant role in its buoyancy in water. Objects with a higher density than water will sink, while objects with a lower density will float. This is due to the fact that the buoyant force is greater on an object with a lower density, causing it to float.

4. Is there a limit to how light an object can feel under water?

Yes, there is a limit to how light an object can feel under water. As the object is submerged deeper, the water above it exerts more pressure, which in turn increases the buoyant force acting on the object. Eventually, the buoyant force will equal the object's weight, causing it to stop feeling lighter and instead feel weightless.

5. Can the shape of an object affect its buoyancy in water?

Yes, the shape of an object can affect its buoyancy in water. Objects with a larger surface area will displace more water, resulting in a greater buoyant force. This is why objects with a larger surface area, such as a boat, can float on water even if they are denser than water.

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