Thread: Please help me with Buoyancy

:? If someone could help me that would be great. I am upgrading and currently I am enrolled in Physics 20. Our teacher decided to throw this at us when we haven't looked at buoyancy at all, so I was hoping someone could help me. This question probably sounds silly but I haven't encountered any of this yet and it is not making sense to me. Here is goes:

Weight of suspended mass (N) - Just under 5N
Weight of mass in water (N) - Just under 4.3 N
Initial volume of water - 1.40 L
Final volume of water - 1.55 L


1. Calculate the buoyant force of water on the mass.
2. From the values above, calculate the volume and weight of water displaced. Compare the weight of the volume of water displaced with the buoyant force acting on the submerged mass calculated in question 1. If the values are different, describe sources of error to account for the difference.


Application:
1. Tim and Susan are floating on an inflatable raft in a swimming pool. What happens to the water level in the pool if both fall off the raft and into the water? Explain your answer.

Lets start with just a few hints and see if it is enough.

I guess that the "final volume" means the volume with the mass inside it and therefore it means the total volume of water and the mass together.

The change in weight means the change in the tension in the string holding up the mass. This changes because the tension balances the other forces on the mass to keep it from falling. Outside the water the tension has to opposed the full weight of the mass, but when it is in the water the boyance force pushes up on the mass to help hold it up and so the tension opposes the weight minus this bouyancy force.

There are no complex formulas to use. It is really very simple and I have practically told you the answer you just have to understand what I said.

For the last question to compare answers: The bouyancy force is equal to the weight of the displaced fluid. So if you know the volume of the mass and the density of water you can figure out the weight of that much water and thus what the bouancy force.

The Application question requires a bit of thought, but you are to assume that Tim, Susan and the raft have stopped moving up and down and are floating in the water.

:-D Thank-you so much. I am going to take a look at this and see what I come up with, I appreciate your time and consideration, thanks.