Magdeburg hemispheres(action and reaction pair)

[asdff529]

My friend said that two horses pull the Magdeburg hemispheres with same force is as same as a horse pulls the Magdeburg hemispheres which is tell on a tree(I may forget the details about the assumption)
Why?
Another Q. about action and reaction

i don't know why the reading isn't 2Mg,but is Mg
Assume the strings are light and the pulley is smooth.

 

[Ken G]

I don't understand what the first question is asking, but you can understand the diagram like this. Put a second spring and meter on the other side also, what does it read? (By symmetry, it reads the same as the first side, call this reading W.) Now put a nail through the box on the top, do any of its readings change? (No it still reads W, the reading is how much the string is stretched, it makes no difference if the box the springs are attached to is nailed down or not.) Now remove the second mass altogether, keeping the box nailed down. Does the reading change? (No, it was nailed down, it doesn't matter if the other mass is there or not.) What is that reading, what must W equal?

 

[bahamagreen]

Analysis is correct.

He is referencing a historic experiment (demonstration) where two teams of horses (8 per side) failed to separate the two halves of an evacuated sphere, the two sides of the sphere having nothing clamping them together but atmospheric pressure.

Someone then suggested that it was unnecessary to use two teams of eight horses, that a single team of eight with the other side chained to a sturdy tree would have tested the same pressure holding the sphere halves together... he was correct, as are you.

 

[Ken G]

Good point, or if one had 16 horses to begin with and wanted to separate the spheres, it would have been better to chain one half to something sturdy, and set all 16 horses to pulling on the other half!