# Physics buoyant force lab.

1. Oct 20, 2013

### astru025

1. The problem statement, all variables and given/known data
Wooden block: Mass: .095. Weight: .931. Volume: 2.03E-4. Density: 467.98. Buoyant force of block: 1.9894.
- Predict how much weight the wooden block can support before sinking.

2. Relevant equations

W= m1g + m2g

3. The attempt at a solution
Not sure where to start. If someone could just point me in the right direction that would be great! I have tried many things that haven't worked...

Here is a picture of the whole lab section:

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2. Oct 20, 2013

### SteamKing

Staff Emeritus
It sure would be nice if this problem had some units attached to it.

3. Oct 21, 2013

### Simon Bridge

Where does the buoyant force come from?
How is it related to the weight?

I'll second the request for units.

4. Oct 21, 2013

### astru025

Units: mass: kg, weight: N, volume: m^3, density: kg/m^3, buoyant force: N.

5. Oct 21, 2013

### Simon Bridge

Thank you :)
What about the principle of Archemedes?

6. Oct 21, 2013

### astru025

I don't know that... It is no where in my notes

7. Oct 21, 2013

### astru025

The equation I'm looking to use is W=m1g + m2g and I need to find what m2 is. I'm not sure where to begin plugging in for this equation though. Using the above info maybe someone could help me?

8. Oct 21, 2013

### Simon Bridge

If only you had access to some sort of searchable database where you could just enter search terms and get lists of possible related articles that you could use when your notes fail you? If only someone would make such a thing available for free through any computer?

I find it hard to believe that you have just done a lab on buoyancy without being given a definition of "buoyancy".
What does the term "buoyancy force", used in the lab, mean to you?

9. Oct 21, 2013

### astru025

Archimedes principle: buoyant force= weights of displaced fluid

10. Oct 21, 2013

### Simon Bridge

Well done:
specifically, an object floats when the buoyancy force is equal to it's weight.

This means: a floating object displaces an amount of fluid equal to it's own mass - but a sunk object, or an object just about to be sunk, displaces fluid equal to it's own volume.

So - what is the buoyancy force on the block when it is as pictured.

Per your question: how much extra mass must you pile on, for the block to float with it's top surface exactly level with the top of the water?

11. Oct 22, 2013

### astru025

.105 kg was my final answer which proved tone correct. Thank you very much.

12. Oct 22, 2013

### Simon Bridge

Well done.

By "correct" do you mean it matched some model answer or that it matched the result of the experiment?
(Or both?)

13. Oct 22, 2013

### nasu

Isn't the buoyant force given in the OP?
It says "Buoyant force of block: 1.9894. ".
As well as the block's weight. You just need to subtract the two.
Unless these things are not given.

14. Oct 22, 2013

### Simon Bridge

That's right - all that was needed.
To get there requires realizing what "buoyant force" actually means...