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Joined: September 30 2006
Location: Pearland
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Points: 64367
Posted: March 15 2014 at 04:22
I caught a few minutes and it seemed interesting but I didn't have the patience for the debut, too talky and backgroundy. Maybe it'll liven up as they progress.
Joined: June 10 2011
Location: Colorado, USA
Status: Offline
Points: 4671
Posted: March 15 2014 at 07:50
I was underwhelmed by the new Cosmos. I like NdT (Stephen Colbert's favorite guest), and I'm not particularly knowledgeable about science, but it seemed a bit too 'dumbed down' (not really the phrase I'm looking for, but I can't think of another). Hopefully will get better.
Joined: August 11 2005
Location: Philly
Status: Offline
Points: 15783
Posted: August 13 2014 at 07:11
I'm not sure if you're asking or this is supposed to be a puzzle. So my answer is in white text below:
Assuming the strings are mass less, I would imagine that the right hand side would fall. It should be independent on the weight of the two balls, only depending on their volume and the fact that the left hand side will float while the right hand side will not. Essentially what's pressing on the scales on both sides is the weight of the water and the pressure of the displaced water by the balls. However, on the left we will have an opposing contribution due to the tension in the string since it is connected to the scale. The left hand side should be lighter.
"One had to be a Newton to notice that the moon is falling, when everyone sees that it doesn't fall. "
Joined: February 10 2010
Location: Barcelona Spain
Status: Offline
Points: 5093
Posted: August 13 2014 at 07:22
^ it was meant as a puzzle. I found it somewhere and I have to admit that although I think I know the answer, I do not know it for sure. I will for the moment refrain from commenting on your solution to see if others have interesting input, I would say that your outcome seems correct to me but not the reasoning.
I am not sure either, so I give my comment in background color. Anyone who is interested can select the text (i.e. by pressing Ctrl-A) to read it:
On the right hand side, the steel ball and the string that suspends it, do not add weight to the scale because the gravitational force is neutralized. On the left hand scale, the weight of the string and the air inside the ball add a little weight (but there is an upward force inside the ball because its density is less than the density of water), so I guess the left hand side will go down.
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Joined: May 13 2007
Location: Europe
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Points: 37575
Posted: August 13 2014 at 08:22
In the interest of fairness I'll not read the hidden explanations until other people have "voted", but I'll simply state: "right-side goes down" and leave it at that for now.
Joined: February 10 2010
Location: Barcelona Spain
Status: Offline
Points: 5093
Posted: August 13 2014 at 09:43
Dean wrote:
In the interest of fairness I'll not read the hidden explanations until other people have "voted", but I'll simply state: "right-side goes down" and leave it at that for now.
It would be however nice to know the reasoning behind the opinions.
Special Collaborator
Retired Admin and Amateur Layabout
Joined: May 13 2007
Location: Europe
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Points: 37575
Posted: August 13 2014 at 09:45
Gerinski wrote:
Dean wrote:
In the interest of fairness I'll not read the hidden explanations until other people have "voted", but I'll simply state: "right-side goes down" and leave it at that for now.
It would be however nice to know the reasoning behind the opinions.
I'll give my reasons when more people have answered, I don't want to sway anyone's thinking (which is also why I haven't peaked at Pat and Robert's explanations).
Joined: August 11 2005
Location: Philly
Status: Offline
Points: 15783
Posted: August 13 2014 at 11:14
someone_else wrote:
I am not sure either, so I give my comment in background color. Anyone who is interested can select the text (i.e. by pressing Ctrl-A) to read it:
On the right hand side, the steel ball and the string that suspends it, do not add weight to the scale because the gravitational force is neutralized. On the left hand scale, the weight of the string and the air inside the ball add a little weight (but there is an upward force inside the ball because its density is less than the density of water), so I guess the left hand side will go down.
This in particular ignores the reciprocal buoyancy force at work on the water.
"One had to be a Newton to notice that the moon is falling, when everyone sees that it doesn't fall. "
Joined: February 10 2010
Location: Barcelona Spain
Status: Offline
Points: 5093
Posted: August 14 2014 at 05:22
^^ OK so here is what I believe is the correct answer, which was also Dean's.
The left container forms an isolated system, with no external forces acting on it (other than gravity of course). The left scale plate holds the mass of the container, the mass of the water and the mass of the ping pong ball.
On the right side the steel ball is held hanging from outside the container system so its mass is not supported by the scale plate. The scale plate holds the mass of the container, the mass of the water and the buoyant force of the ball, which Archimedes told us is equivalent to the volume of water displaced by the ball. Since a ball of water is heavier than the ping pong ball, the scale will tip to the right.
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.
Incidentally, it is incorrect to think that the ping pong ball will exert a force upwards on its container due to its tendency to float. They form part of the same system and as they say, you can not lift yourself up by pulling from your bootstraps.
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