EK PHY lecture question 99

[DAKAZA]

I know the brick's s.g. is 1.4 but I don't understand why the brick needs to displace a volume of water 1.4 times it's own volume. I've been trying to understand this for a long time...frustrated

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[HawkeyePostOp]

It needs to displace 1.4x the volume to float.

Because water pushes up with a force proportional to the volume displaced times the mass of water.

Imagine a hypothetical block of water in water. The surrounding water is pushing up with a force of (how much a block of water would weigh). Since a block of water weighs one block of water, the forces cancel out and the "block" doesn't sink. If it were heavier, it would sink. To not sink, it would need to be surrounded by air or styrofoam or something relatively massless to displace more volume, thus increase upward pressure from the water.

 

[DAKAZA]

It needs to displace 1.4x the volume to float.

Because water pushes up with a force proportional to the volume displaced times the mass of water.

Imagine a hypothetical block of water in water. The surrounding water is pushing up with a force of (how much a block of water would weigh). Since a block of water weighs one block of water, the forces cancel out and the "block" doesn't sink. If it were heavier, it would sink. To not sink, it would need to be surrounded by air or styrofoam or something relatively massless to displace more volume, thus increase upward pressure from the water.

Thanks so much. The hypothetical block of water really helped. I finally understand