Physics fluids - buoyant principle help

In summary, the problem is asking for the weight of the man's body when submerged in water, with 90% of his body under water. This can be calculated by finding the apparent weight, which is equal to the weight minus the buoyant force. Since the man is floating and staying put, the buoyant force is equal to the weight of water displaced, which is proportional to 90% of his volume. Therefore, the apparent weight is zero and the man's weight is mg, or the force due to gravity. In this case, the man is not sinking or rising, so the gravitational force and the buoyant force are in equilibrium.
  • #1
mindhater
6
0
A problem states that a man with a mass of 55 kg floats with 90% of his body under water.

I jus need to find the weight of his body when submerged in water...i'm was thinking 90% of 55 kg, but i wasn't sure if that was correct...
 
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  • #2
Apparent weight is the weight minus the buoyant force: W-BF

Buoyant force is equal to the weight of water displaced. The BF is equal to the weight of the water displaced. If 90% of the dude's volume is displacing water, that force caused by the displacement is proportional to 90% of his volume. Furthermore the guy is staying, put he's not going up or down...think about what this means, you don't need to know the volume or even the weight for this problem.
 
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  • #3
This looks like a trick question.

"weight" is an ambiguous term: it could mean the "force due to gravity," in which case the man's weight is mg. "weight" could mean (as was stated) the apparent weight which is mg-BF. IF this is the case, then it is zero because the man is floating.

If you are floating with some of your body above the surface, then the bouyant force is equal to the density of water times "g" times the "submerged volume" of the body; the gravitational force is still the entire mass of the whole body times "g." When floating, these forces are in equilibrium.
 

1. How does the buoyant principle work in physics fluids?

The buoyant principle states that the upward force on an object immersed in a fluid is equal to the weight of the fluid displaced by the object. This is known as Archimedes' principle.

2. What factors affect the buoyant force on an object in a fluid?

The buoyant force depends on the density of the fluid, the volume of the object, and the depth at which the object is submerged. It is also affected by the gravitational force acting on the object.

3. How does the buoyant force determine whether an object will float or sink in a fluid?

If the buoyant force on an object is greater than the weight of the object, it will float. If the weight of the object is greater than the buoyant force, it will sink. Objects with a density less than the density of the fluid will float, while objects with a density greater than the fluid will sink.

4. Can the buoyant force be used to calculate the volume of an irregularly shaped object?

Yes, the buoyant force can be used to calculate the volume of an irregularly shaped object by submerging it in a known fluid and measuring the amount of fluid it displaces. This is known as the displacement method.

5. How is the buoyant force related to the concept of density?

The buoyant force is directly related to the density of the fluid in which an object is submerged. A less dense fluid will exert less buoyant force on an object compared to a more dense fluid. This is why objects with higher densities, such as rocks, will sink in water but float in less dense fluids like oil.

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