Homework Help: 2 Bricks underwater

1. May 2, 2006

jcais

Hello,

There are two bricks underwater. Brick A is closer to the surface and Brick B is further down below.

Which Brick would be hardest to keep in place?

I said Brick B becuase there is more pressure (H20) on it forcing it down + gravity.

Brick A just has gravity and a little bit of water pressure.

Am I right or wrong?
My friend agreed with me, but he said we could be wrong because it seems to easy....like a trick question.

2. May 2, 2006

Curious3141

It is a trick question. What do you know of Archimedes' principle ?

3. May 2, 2006

jcais

In physics, buoyancy is an upward force on an object immersed in a fluid (i.e. a liquid or a gas), enabling it to float or at least to appear to become lighter. If the buoyancy exceeds the weight, then the object floats; if the weight exceeds the buoyancy, the object sinks. If the buoyancy equals the weight, the body has neutral buoyancy and may remain at its level. ...

So, I am guessing that they are equal. Oh well, 5 points off of my test. :grumpy:

4. May 2, 2006

Curious3141

Yeah, the expected response is probably that they are equally "difficult/easy to keep in place". I do sympathise with you in that the wording of the question is poor - I would merely have asked which brick has a greater net force acting on it, which would've been clearer and more to the point.

Archimedes' principle states that a body immersed in a liquid experiences a buoyant force equal to the weight of the fluid it displaces. For a particular object with a particular volume, this weight of fluid will be the same, hence the buoyant force will be the same, no matter where it is in the fluid medium (even its orientation doesn't matter).

5. May 3, 2006

Staff: Mentor

Now I'm curious about something. What if each brick is resting on a flat surface at the different depths? Is the answer still the same? I can see that the force will be the same if there is water (and hence water pressure force) on the underside of each brick, but what if there is no water? Then the downward force on each brick would seem to be different, since the water pressure is higher at the greater depth.... Help me out here?

6. May 3, 2006

Curious3141

In the case of rigid supports in contact with the underside of the bricks, I believe the situation will be different. Let's say there're two strong flat pedestals attached to rigid beams that're firmly planted in a foundation in the bed of the water body. The two pedestals are of different heights. In this case, I would expect that the water pressure on the top surface is unbalanced (the sides cancel out). The brick that is deeper down (on the shorter pedestal) has a greater unbalanced force acting to push it downward, leading to greater stability and greater "apparent weight" if one tries to lift it off the pedestal.

7. May 3, 2006

Staff: Mentor

So now you don't have to worry about losing any points on the question! The question was incomplete. If the bricks were sitting in silt or on coral, the upward force to lift them would be the same, independent of depth. If they are sitting on an artificial flat surface where there is no water between then and the flat surface, then the force required to lift them increases with depth. Go to your prof now and get some extra credit points added! And be sure to mention PF as a source.

8. May 3, 2006

Curious3141

See, this is what happens when you allow "normal" people - non-obsessive-compulsives - to set questions.

I'm terribly obsessive about the wording of any questions I pose or set. Having someone else point out an ambiguity or loophole would be unbearable. I guess I'll never make a good teacher or test-setter, it would just take me too long to get the wording just right. Also, the questions might end up being longer than the answers!! :rofl:

9. May 3, 2006

Staff: Mentor

This all brings up an interesting thought experiment....

So if it's true what Curious and I are saying about the brick being harder to pull off of a flat surface at deeper depths, then it should also be true that when you are deep enough, the brick will stick to the flat surface even if the surface is tilted vertical, and even at some depth if the surface is inverted....?

So take a smooth-sided brick and place it on a piece of smooth-sided panel while you are out of the water at the surface. The smooth surfaces are in intimate enough contact so that water will not be able to get between the brick and the panel. Now descend to some depth d. Water pressure starts at 1ATM at the surface, and increases by 1ATM for every 10m or so of depth. Can we find some minimum depth d where we can turn the panel over and let go of the brick, and it will still stay stuck to the panel?

10. May 3, 2006

Staff: Mentor

Should be simple enough: The force due to water pressure must equal the weight of the brick. When the brick/panel is upside down, only the bottom surface of the brick counts.

Edit: Now that I think about it, if the seal between brick and panel were airtight, air pressure alone would probably hold the brick in place.

Last edited: May 3, 2006