The Science Thread

Gerinski wrote:   Notice that if the ping pong ball was being pushed and kept submerged from the top from outside the container, say by a wire rod with a small cupped end at the bottom, then the scale would stay balanced.

1) I see the tension in the string keeping the ball in equilibrium to be an important part of the analysis.

EDIT: Also mean to say, you are correct the displaced water does not factor on the left. 

EDIT Again: A google image searched turned up a solution where someone was nice enough to draw the diagram and write out the equations. So definitive answer I suppose here.


Edited by Equality 7-2521 - August 14 2014 at 06:57

Dean wrote: As I love to disagree, I shall continue to do so.  Both objects have the same tendency to float by the same degree, both are exerting the same downward force on the scale. That is what Pat has said in the second sentence you have highlighted The steel ball does not float because the force of gravity acting upon its mass is far greater than the buoyancy force pushing it up. That is what Pat has said in the first sentence you have highlighted. The tension in the string that keeps the ping pong ball submerged is exerting an equal and opposite force on the ping-pong ball to counter-act the weight of water the ball displaces. That is what he has said in the third highlighted sentence. The left-hand side is lighter than the right, it is not lighter because it is trying to pull itself up by its own bootlaces (which is what you have interpreted).

You responded on my behalf much more eloquently than I did. Thank you.

Equality 7-2521 wrote: You responded on my behalf much more eloquently than I did. Thank you.

Yeah with the obvious simplifying assumptions why would anyone think anything except (b).

Oh no. 'Hypothetical' implies that, as Pat said, certain simplifying assumptions are made and the bounds of the answer are contained within the limiting conditions set by the question. The primary assumption inherent in the formulation of the question is that the environmental conditions can, hypothetically, support the subject of the experiment. The thought-experiment is only concerned with the effects of gravity. Under those predefined conditions and implied assumptions (b) gives the correct answer but not the exact reason. 

Edited by Dean - August 15 2014 at 07:58

Because every multiple choice has a silly answer?

The assumption said that "at any given distance from the center towards the surface, the average density is the same in every direction". This is not saying that density is the same at any given distance, but only that there are no noticeable concentrations of mass which could pull you in some direction more than to others, that the Earth is formed of concentrical shells of a certain density each. But unless explicitly stated otherwise, logic says that density increases with depth.

It's not a matter of being bad loser (which undoubtedly I am - who the hell wants to be a good loser?), it's the whole smug nature of "trick questions" I dislike. The ball and balance problem isn't a trick question and no simplifying assumptions are required to solve it.