What is the density of the wood?

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Homework Help Overview

The problem involves determining the density of a block of wood that floats in both water and oil, with specific volumes submerged in each liquid. The context is related to fluid mechanics and buoyancy principles.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the relevance of the submerged volume in water and oil to find the wood's density. Some express uncertainty about the approach, while others suggest using free-body diagrams and Newton's second law to analyze the forces acting on the block.

Discussion Status

There is an ongoing exploration of methods to solve the problem, with some participants providing guidance on using free-body diagrams. However, there is no explicit consensus on the best approach, and confusion remains regarding the density of the oil.

Contextual Notes

Participants note the known density of water and express the need for additional information regarding the oil's density, which is not provided in the problem statement.

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Homework Statement


A block of wood with volume V floats in water with 0.68V submerged. In oil the block floats with 0.86V of its volume submerged.

Find the density of the wood.


Homework Equations



\rho = M/V




The Attempt at a Solution



I'm not really sure about how to attempt to solve this problem. I know the density of fresh water and that's about it. Can anyone help?
 
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Find the density of the WOOD, not of the oil? You don't need to know about the oil for that. Just draw a free-body diagram for the block of wood and write out Newton's second law.
 
Oh ok! I got it, thanks!
 
Ok I got the density of the wood but I could not find the density of the oil.

For the wood this is what I did..

density of water = 1 g/cm^3
and since the volume is .68 submerged then you divide the .68 by 1g/cm^3

but this method didn't work for the oil
 
Dividing 0.68 by 1 only worked because the density of water happened to be 1 g/cm^3, and multiplying by 1 is equivalent to dividing by it. If the density was anything else, your method would have given the wrong answer.

If you're getting confused, you should really draw a free-body diagram and write out Newton's second law instead of trying to take shortcuts. FBD + Newton's second law is a simple, elegant, and intuitive way of approaching this problem that depends only on first principles. For this problem, draw two FBD's, one for the block when it's in oil and one for the block when it's in water. Then solve the equations.
 

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