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Pressure and upthrust in fluids: 7.2 Some 'kitchen sink' experiments on density in the home!

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7.2 Some 'kitchen sink' experiments on density in the home!

Density experiments using an apple, ice and wooden blocks in water.

A The apple has a density of ~900 kg/m3 and floats on water, of density 1000 kg/m3

The upthrust of the water is equal to the weight of the apple, the forces balance and the apple floats.

The weight of the water displaced by the apple equals the weight of the apple.

You can see that roughly 9/10ths of the apple is immersed in the water - its similar force (see F below).

B This particular potato has a density greater than water and sinks, so we have no idea what its density might be.

The upthrust of the water is less than the weight of the potato, the forces are unbalanced and the potato sinks.

The potato is unable to displace enough water to equal its weight, so it can't float.

The weight of the water displaced by the potato does NOT equals the weight of the potato, so it sinks.

C The block of wood has density of ~500 kg/m3 (half of that of water) and floats (it toppled over!) - its density is half that of water and is only half submerged - can you reason out why it is only half submerged?

The upthrust of the water is equal to the weight of the block, the forces balance and the block floats.

BUT the block only has to displace half of its own volume to displace enough water to equal its own weight and the upthrust.

D A block of wood of density ~1000 kg/m3, the same density as water, so it floats with its upper surface coincident with the surface of water.

It couldn't be any more immersed in the water without sinking!

E Another block of wood of density ~600 kg/m3, so more of it is immersed in water than the other less dense bloc of wood above.

F I made a small 'barrel' of ice and placed in the beaker of water. Since ice has a density of 917 kg/m3, it floats on water.

You can just about see that about 10% of the 'mini-iceberg' is above the surface of the water.

Its the 90% of an iceberg below the surface that is the main danger to ships (the Titanic!), not the 10% you can see!

As far as I know, ice is the only solid substance that is less dense than its liquid form.

The molecules of ice form a quite widely spaced crystal lattice so that on average the molecules are further apart than they are in the liquid, despite the contrast between the ordered structure of ice and the randomness of liquid water molecules.

If you do advanced A level chemistry you go into the details of hydrogen bonding and the structure of water - its quite interesting, but more advanced stuff than required here!)

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Keywords, phrases and learning objectives for simple float or sink experiments in the home

Be able to explain some simple kitchen sink home experiments on density in home, ice, fruit like apples and wooden blocks - will they float or sink?


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