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|#7 - I think what the bouyant force of the ping pong ball would be … [+] (7 new replies)||11/23/2013 on Problems.||-5|
#9 - KazumaKyu (11/23/2013) [-]
... The buoyancy of the ball has no affect whatsoever. You are implying that the upward force generated by an object's buoyancy can be somehow transferred to the container the buoyant object is floating in.
Look, it was in my explanation above--if you've got a beaker of water with a ping pong ball floating in it, it will be heavier than a similar beaker filled with a similar amount of water that doesn't have a ping pong ball floating in it. The fact that the ball is tethered to the bottom of the beaker is unimportant, and is only there to confuse you with extraneous data. The whole scale isn't submerged, so the fact that the ping pong ball has buoyancy is completely irrelevant.
If the entire setup were under water, then I would agree that the ping pong ball's buoyancy would outstrip its mass, but since it isn't it's unnecessary information that cannot be factored in if you want to come to a conclusion that isn't wrong.
You have to stop and think about what exactly the ping pong ball is straining against. If we were measuring the mass of an object submerged in water that had a ping pong ball tied to it, I would completely agree that the ball's buoyancy would create the illusion that the object was suddenly composed of less mass. But because the object whose mass is being measured (the beaker) is not also submerged in a substance with greater density than the air within the ping pong ball, the ball's buoyancy cannot be taken into account.
How about this, let's scale it up: let's say you're tethered to the bottom of an aquarium, and you've got a lungful of air. Because of that lungful of air, your body has a certain buoyancy. Claiming that the ping pong ball would effectively lessen the mass of the beaker is exactly the same as saying an aquarium would be lighter if you were tied to its floor.
#17 - soloman (11/23/2013) [-]
Actually he's right, but wrong. The buoyant force and tension would affect the container, but in opposite directions of how he drew them. Newtons second law. The force has to have a reaction, and for the buoyancy this is the water, and for the tension it's the bottom of the container. Both of which will affect the reading of the scale.
The reason the container would get heavier with a ping pong ball floating in it, as you mentioned in your first comment, is because the reaction in the water to the buoyant force of the ball.
So you're both right, you more so than he, but he has a point, even if it was redundant.
#22 - soloman (11/23/2013) [-]
The force is exerted on the water, which in turn the water must exert on something else to remain at 0 net force. So the force is exerted on the water, which is exerted on the container, which is then exerted on the scale. If there's external forces, it's not a closed system. The water can't just apply forces to the ball without affecting anything else.
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