# Confused abt buoyancy concepts

1. Oct 16, 2008

1. The problem statement, all variables and given/known data
Imagine a man in a boat holding a brick. He toss the brick in the water. What will happen to the level of the water when the brick toss in it.

2. Relevant equations
None

3. The attempt at a solution
I have arrived on 3 solution thats seems to be logical to me but i dunno which is the correct one. Please help to explain to me which one is correct.
The first one is the water level will stay the same due to brick, boat and man already reflected in the water level. The water level is the same irregardless if the brick is in the boat or water.
The second one is the water level will decrease. While the brick is still in the boat, the amount of water being displaced by the brick is equal the mass of the brick. When he chucks it in the water, the boat rises, as amt of water due to the mass of brick is no longer being displaced by the boat. When the brick is in the water,it displace less water because it sinks. Less water displaced by the brick means that there is a decrease in water level.
The last one is that the level of water will rise because an additional mass, which is the brick is introduced in the water.
Arghhhh too confusing alrdy. =(

2. Oct 16, 2008

### tiny-tim

As you know, I'm not allowed to tell you which is the right answer.

But I can suggest a shorter and more logical approach …

when the brick is floating, it displaces its (what?),

and when the brick is immersed, it displaces its (what?)

3. Oct 17, 2008

weight? can guide me more?

4. Oct 17, 2008

### Staff: Mentor

Two different things are displaced in both cases!

5. Oct 17, 2008

when the brick is floating, it displaces its (own weight of water),
and when the brick is immersed, it displaces its (own volume)?

6. Oct 17, 2008

### tiny-tim

eureka!

Woohoo!

And so the water level … ?

7. Oct 17, 2008

less water is displaced by the brick , which means that water level will decrease..am i right?

8. Oct 17, 2008

### tiny-tim

Yes!

Except … there's nothing actually in the question that tells you how dense the brick is, is there?

So strictly you ought to make some comment about the density of the brick!

9. Oct 17, 2008

then if the brick is less dense than the water (assume that this kind of brick exists), will the water level increase as a result of more water being displaced by the brick?

10. Oct 17, 2008

### tiny-tim

Think!

If you throw something in the water that's less dense than the water, then it will … ?

11. Oct 17, 2008

it will float. more water is displaced by the brick and water level rises?

12. Oct 17, 2008

i have another query abt buoyancy. Throw a iron bar in a pail of water. The water level will rise. But why doesnt the water level decrease as in the brick example since iron bar displaces its own volume?

13. Oct 17, 2008

### tiny-tim

Yes.
Why?

14. Oct 17, 2008

hmmm....when an object floats, it will displace some water. can guide me further?

15. Oct 17, 2008

### tiny-tim

Yes … but will it displace any more water than when it was in the boat?

16. Oct 17, 2008

ahhhhh...i get it now.. the water level remains the same as the amt of water displace is the same

17. Oct 17, 2008

hmm how abt the iron bar scenario in #12

18. Oct 17, 2008

### tiny-tim

wot #12?

19. Oct 17, 2008

opps it means post 12, which is
"i have another query abt buoyancy. Throw a iron bar in a pail of water. The water level will rise. But why doesnt the water level decrease as in the brick example since iron bar displaces its own volume? "

20. Oct 17, 2008

### tiny-tim

ooh, I missed that … it came in while I was replying to post #11!

It's because, with the brick, the contest was between displacing its own weight and displacing its own volume.

But, with the iron bar, it wasn't displacing anything to start with, so … no contest!