- #1
elvinc
- 10
- 0
Hi,
Can someone explain to me what happens when an object sinking in a liquid, hits the bottom of the container holding the liquid. I accept this is an "ideal" situation but it has turned up something I don't understand.
So, say we have a box with rectangular faces (a cuboid) that is immersed in a liquid in a larger container. The liquid is less dense than the material the box is made of, say a lead box placed in water. The box experiences upthrust equal to the weight of the volume of water displaced. This upthrust happens because the pressure on the bottom face of the box is greater than the pressure on the top face (P = hrg) . As the top and bottom faces of the box have equal area we can replace pressure by force and see there is an upthrust (force) on the box. However as the box is made of material more dense than water the weight of the box will exceed the upthrust and the box will sink in the liquid. So far, so good. Now assume the surface of both the immersed box and the floor of the container of liquid are "perfectly flat". When the box settles on the bottom of the container there is no longer any water underneath the box so there is no upward pressure and no upward force. Any forces on the sides of the cuboid will be symmetrical and opposite and will cancel - therefore not contributing to upthrust. Therefore as the box settles on the bottom of the container it will experience a sudden increase in apparent weight as the upthrust has disappeared. However, this just "feels" wrong to me - but I can't explain why. I would expect the measured weight (actual weight - upthrust) to be almost identical just before the box settles on the bottom of the container and as it comes in contact with the bottom of the container.
Any ideas?
Thanks
Clive
Can someone explain to me what happens when an object sinking in a liquid, hits the bottom of the container holding the liquid. I accept this is an "ideal" situation but it has turned up something I don't understand.
So, say we have a box with rectangular faces (a cuboid) that is immersed in a liquid in a larger container. The liquid is less dense than the material the box is made of, say a lead box placed in water. The box experiences upthrust equal to the weight of the volume of water displaced. This upthrust happens because the pressure on the bottom face of the box is greater than the pressure on the top face (P = hrg) . As the top and bottom faces of the box have equal area we can replace pressure by force and see there is an upthrust (force) on the box. However as the box is made of material more dense than water the weight of the box will exceed the upthrust and the box will sink in the liquid. So far, so good. Now assume the surface of both the immersed box and the floor of the container of liquid are "perfectly flat". When the box settles on the bottom of the container there is no longer any water underneath the box so there is no upward pressure and no upward force. Any forces on the sides of the cuboid will be symmetrical and opposite and will cancel - therefore not contributing to upthrust. Therefore as the box settles on the bottom of the container it will experience a sudden increase in apparent weight as the upthrust has disappeared. However, this just "feels" wrong to me - but I can't explain why. I would expect the measured weight (actual weight - upthrust) to be almost identical just before the box settles on the bottom of the container and as it comes in contact with the bottom of the container.
Any ideas?
Thanks
Clive