Physics Conceptual Question: Buoyancy?

[Goodyearkl]

Homework Statement

A fisherman and his young daughter are in a boat on a small pond. Both are wearing life jackets. The daughter is holding a large floating helium filled balloon by a string. Consider each action below independently, and indicate whether the level of the water in the pond R-Rises, F-Falls, S-Stays the Same, C-Can't tell. (If in the first the level Rises, and in the second it Falls, and for the rest one Cannot tell, enter RFCCCC)

A) The daughter pops the balloon.
B) The fisherman lowers the anchor and it hangs vertically, one foot above the bottom of the pond.
C) The fisherman lowers himself in the water and floats on his back
D) The fisherman fills a glass with water from the pond and drinks it.
E) The fisherman knocks the tackle box overboard and it sinks to the bottom.
F) The daughter gets in the water, looses her grip on the string, letting the balloon escape upwards.

Homework Equations

Archimedes Principle

The Attempt at a Solution

A) S because the balloon is above the water so isn't really adding any weight.
B) S the anchor isn't touching the water
C) C would the glass really make all that much a difference considering the volume of water involved...it's a pond.
D) S He's floating so the water should't change
E) R the water displaced is less than the weight of the object, the object will sink
F) S Same as the father?

 

[alphysicist]

Hi Goodyearkl,

Here are some thoughts on your reasons:

Homework Statement

A fisherman and his young daughter are in a boat on a small pond. Both are wearing life jackets. The daughter is holding a large floating helium filled balloon by a string. Consider each action below independently, and indicate whether the level of the water in the pond R-Rises, F-Falls, S-Stays the Same, C-Can't tell. (If in the first the level Rises, and in the second it Falls, and for the rest one Cannot tell, enter RFCCCC)

A) The daughter pops the balloon.
B) The fisherman lowers the anchor and it hangs vertically, one foot above the bottom of the pond.
C) The fisherman lowers himself in the water and floats on his back
D) The fisherman fills a glass with water from the pond and drinks it.
E) The fisherman knocks the tackle box overboard and it sinks to the bottom.
F) The daughter gets in the water, looses her grip on the string, letting the balloon escape upwards.

Homework Equations

Archimedes Principle

The Attempt at a Solution

A) S because the balloon is above the water so isn't really adding any weight.

But the balloon is affecting how much weight the water has to support.

B) S the anchor isn't touching the water

I'm not saying your answer is wrong, but I believe they mean the anchor is lowered into the water but not allowed to touch the bottom of the pond.

C) C would the glass really make all that much a difference considering the volume of water involved...it's a pond.

This was supposed to be for part D, right? It won't make much of a difference (none of these will), but would it make any difference at all?

D) S He's floating so the water should't change
E) R the water displaced is less than the weight of the object, the object will sink

Your reasons are right (if you mean that the weight of the water displaced by the object is less than the weight of the object); why would that make the water rise?

 

[nlightner]

Your attempt at a solution is mostly correct. Here is a more detailed explanation:

A) C: When the balloon is popped, the volume of the balloon decreases and the weight of the balloon decreases as well. However, the balloon is still attached to the string and is still in the water, so the overall effect on the water level is negligible.

B) S: The anchor is not touching the water, so it is not displacing any water and therefore not affecting the water level.

C) F: When the fisherman lowers himself into the water, he is displacing a certain volume of water equal to his own weight. This causes the water level to rise slightly. However, the effect is small because the fisherman's weight is spread out over a larger area compared to the balloon in the previous question.

D) S: Drinking the water from the pond does not affect the volume or weight of the water in the pond, so the water level remains the same.

E) R: The tackle box is heavier than the water it displaces, so it sinks to the bottom. This means that the volume of water it was displacing is now free to rise, causing the water level to rise.

F) S: Similar to the father in question C, the daughter's weight is spread out over a larger area compared to the balloon, so her entry into the water does not significantly affect the water level.