How many people before it sinks

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Homework Help Overview

The discussion revolves around a problem involving buoyancy and weight distribution on a raft made of logs. Participants are exploring how many people can stand on the raft before it begins to sink, given specific dimensions and densities of the materials involved.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the relationship between the weight of the raft, the weight of the displaced water, and the total weight of the people on the raft. There are inquiries about defining variables and understanding the forces at play, such as buoyancy and weight.

Discussion Status

Some participants have offered guidance on calculating the weight of the logs and the volume of water displaced. There is an ongoing exploration of the conditions under which the raft will begin to sink, with multiple interpretations of the problem being discussed.

Contextual Notes

Participants are encouraged to define their variables clearly, and there is a suggestion to draw a free body diagram to aid in understanding the forces involved. The problem is framed within the constraints of typical physics homework, focusing on buoyancy principles.

superjen
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A raft is made of 9 logs lashed together. Each is 29.1 cm in diameter and has a length of 5.7 m. How many people can the raft hold before they start getting their feet wet. Assume the average person has a mass of 69.3 kg and that the density of water and wood is 1000 kg/m3 and 600 kg/m3, respectively

All i know iss

FB = mpg + mrg
i don't really know how to figure it out from here!
 
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A floating object displaces water equal to it's weight.
So how much mass of water would be displaced if the logs were completely submerged (hint volume of logs and density of water)
Now how much do those logs alone weigh - and so how much extra weight of people could be added.
 
can you define your variables, it will help us to answer your question too? ie. what is FB and mpg and mrg?

Remember that the force of bouyancy is related to the volume of the liquid that is displaced.

Don't forget to find out the mass (and weight) of all the logs.

Drawing a diagram (Free body diagram) with force arrows will also help.
 
mgb_phys beat me to it. :smile:
 
People will start to get their feet wet when the logs are just underwater and the people aren't. So the upthrust due to the logs will be equal to the weight of the logs and of the people standing on them.

Upthrust = weight of water displaced = log volume * water density

DownForce = weight of logs + weight of people

Our condition is Upthrust = DownForce
 

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