Buoyancy: 3 solid things have a different position in the water

[Yoseph Santoso]

Homework Statement

There is three things P, Q, R. The density of P is 2000 kg/m³, Q is 1000 kg/m³, R is 2500 kg/m³. If all of them are put into the water having density 1000 kg/m³, the correct position of the things are shown by following picture ...
http://[ATTACH=full]200216[/ATTACH]

[h2]Homework Equations[/h2]
When ρ [SUB]thing[/SUB] < ρ [SUB]water[/SUB] : the thing is floating
When ρ [SUB]thing[/SUB] = ρ [SUB]water[/SUB] : the thing is float halfway
When ρ [SUB]thing[/SUB] > ρ [SUB]water[/SUB] : the thing is sinking

 

[gneill]

Hi Yoseph,

Welcome to Physics Forums.

1. One of your images is "broken" (the one in the problem statement section). Can you try to load it again?
2. Your problem statement doesn't include a question. What are you trying to find out?
3. Your post lacks the "attempt at a solution" section. Note that you must show us what you've tried before help can be given.

 

[Yoseph Santoso]

There are three things P, Q, R. The density of P is 2000 kg/m3, Q is 1000 kg/m3, R is 2500 kg/m3. If all of them are put into the water having density 1000 kg/m3, which one is the correct position of the things? Picture A or Picture B (see attachment)?

 

[gneill]

which one is the correct position of the things? Picture A or Picture B

What is your choice? Can you apply your relevant equations to definitely rule out one of the choices?

 

[Yoseph Santoso]

Thanks for your reply. This is my homework that I don't know what's the right answer, so I want your helping to solve this problem.

 

[gneill]

Thanks for your reply. This is my homework that I don't know what's the right answer, so I want your helping to solve this problem.

We can help you find a solution, but you'll need to put in some effort.

Look at image A:

Check each of the objects P, Q, R against your relevant equation criteria. What do you find?

 

[Yoseph Santoso]

Sorry this is my attempt at a solution :
At first I chose picture B because things P and R are solid, so they will go to the bottom of the container. Whereas, my teacher choose picture A.

 

[gneill]

Sorry this is my attempt at a solution :
At first I chose picture B because things P and R are solid, so they will go to the bottom of the container. Whereas, my teacher choose picture A.

You mean that P and R are both more dense than the fluid, so they should both sink to the bottom. That is true, and I agree with your choice of B as being correct.

 

[Yoseph Santoso]

We can help you find a solution, but you'll need to put in some effort.

Look at image A:
View attachment 197164
Check each of the objects P, Q, R against your relevant equation criteria. What do you find?

I think P position is impossible, it must be in the float halfway or sink. On the other hand, my teacher say that the correct answer is A. It's to make sure that the density of R is greater than P so that R position is lower than P position. According to you, does my teacher's reason make sense?

 

[gneill]

I think P position is impossible, it must be in the float halfway or sink. On the other hand, my teacher say that the correct answer is A. It's to make sure that the density of R is greater than P so that R position is lower than P position. According to you, does my teacher's reason make sense?

No. If an object's density is greater than that of the fluid it will sink to the bottom. Both P and R have densities that are greater than that of the fluid, so they must sink to the bottom.

The only time an object can "hover" in the fluid without moving up or down is when it has neutral buoyancy. That means its density is equal to that of the fluid. That only holds for one of the objects.

 

[Yoseph Santoso]

No. If an object's density is greater than that of the fluid it will sink to the bottom. Both P and R have densities that are greater than that of the fluid, so they must sink to the bottom.
The only time an object can "hover" in the fluid without moving up or down is when it has neutral buoyancy. That means its density is equal to that of the fluid. That only holds for one of the objects.

Okay thanks gneill, now I confident with my answer and learn more from you.

Let me ask one more question . I ever made an experiment about neutral buoyancy with water, egg, and salt. I wanted to put egg hover in the water by adjusting the density of the water. When the egg sank, I added salt liquid little by little and when the egg floated, I added water little by little until it hover in the middle. But after few minutes, that egg couldn't stable in the middle anymore, it would end with sink or float position. Could I make the egg hover at least for several minutes?

 

[gneill]

Okay thanks gneill, now I confident with my answer and learn more from you.

Let me ask one more question . I ever made an experiment about neutral buoyancy with water, egg, and salt. I wanted to put egg hover in the water by adjusting the density of the water. When the egg sank, I added salt liquid little by little and when the egg floated, I added water little by little until it hover in the middle. But after few minutes, that egg couldn't stable in the middle anymore, it would end with sink or float position. Could I make the egg hover at least for several minutes?

I think it would be difficult to achieve exactly neutral buoyancy conditions (net force exactly zero), and hard to maintain over time. It's a bit like trying to balance a pencil vertically on its point.

There are physical processes going on at the same time that can move your carefully balanced system out of balance. For example, if the egg and water and container and room air are not precisely the same uniform temperature, as they exchange heat the water and egg will expand or contract a tiny bit and by different amounts, altering the displaced volume and changing the buoyancy.

 

[Yoseph Santoso]

Okay, I see my great teacher. Thanks for your clear explanation.