Thread: buoyancy question

We all know that buoyancy is the force that causes ships, wood planks, ice, etc to float on water. We also know that the magnitud of buoyancy is equal to the weight of the displacement fluid.

The question is, does oil displace water? What is the upward force that acts on liquids such as oil when it is floating on water?

Yep oil displaces water. It is heavier than air, lighter than water so it is effectively crushed if you like, and so it spreads out.

Alright, a totally different question. Now, if a cup of water has a mass of 20g, will the weight of the water increase if i add an ice cube that has a mass of 1g? (Assuming that 1kg = 10N)

Theoretically, the buoyancy should cancel out the weight, but if that is the case, the weight of a cup with 20g of water will still weigh the same regardless of the oil added above it? (Sounds absurd)

My facts are screwed up, enlighten me!

Originally Posted by Liam90

Alright, a totally different question. Now, if a cup of water has a mass of 20g, will the weight of the water increase if i add an ice cube that has a mass of 1g? (Assuming that 1kg = 10N)

Theoretically, the buoyancy should cancel out the weight, but if that is the case, the weight of a cup with 20g of water will still weigh the same regardless of the oil added above it? (Sounds absurd)

My facts are screwed up, enlighten me!

The weight of the water with the ice in it will be 21g.
If you add oil to your cup of water the total weight will increase by an amount equal to the weight of the oil. Bouancy is nothing to do with weight (on it's own) it is property of relative density.

If you are assuming that the cup is full to the brim then adding ice will cause an amount of water equal to the weight of the ice to overflow, this would leave the cup, water and ice, weighing (in theory) the same as the original cup and water only, in any practical experiment you can only get close to the theoretical answer.

thanks :-D

Wait! You do know that the weight i was referring to is the reading on the scale of a weighing machine right?

Let me rephrase my question.

A cup of water is put on a weighing machine.
The scale shows that the cup + the water weigh 20g.
A ice with the mass of 1g is put into the cup of the water.
What is the reading on the weighing machine?

Just to confirm. Sorry for the trouble

It's the same principle all over again, the weighing machine itself should in practice make no difference to the weight of the object which it is trying to weigh regardless of whether that weight is increased/decreased whilst it is being weighed (if it is an extremely heavy weighing device then in theory it could increase the weight but I'm being pedantic and the device itself would be calibrated to take this into account anyhow).

if the cup was full of water initially, and we add a wooden block into the cup, the total weight will not change. since the weight of water that is poured out of the cup is equal to the weight of the wooden block.