# Hydrostatic Paradox - weighing issue

In the picture below, both tubes have the same pressure at the bottom. I understand that if they were weighed individually, they would show a different result since the water on the second tube is exerting an upward force on the "roof", removing "excess" force on the scale.

But what if the scale was put inside the tubes (red lines)? Also, the sides of the tubes are mounted onto an immovable wall, preventing any upward force affecting the scale from below. Now since pressure = Force/area, and area and pressure are the same in both, the scales must be subjected to the same forces as well. Will the second scale now show an incorrect result?

• The scale reads the weight of whatever is placed upon it, REGARDLESS of the shape of the container. Mar 12, 2019 at 18:23
• So if the first tube holds 5l and the second 4l of water, then the scales will show 5kg and 4kg? This goes against the two answers so far. Could you add another answer? Or did I just interpret your comment incorrectly? Mar 12, 2019 at 19:43
• Yes, the scales would read 5 kg and 4 kg, if the scales are outside of the containers. I maintain that both scales would read zero if they are submerged inside the containers, as your red lines indicate in the drawing. Mar 12, 2019 at 19:49
• yes, in my scenario the scales are inside the containers. why would they read zero? isn't there a force applied to the scales? Mar 12, 2019 at 20:15
• There is a force applied to the scale from the liquid that is above it, but there is another force applied to the bottom of the scale from the liquid that is below it. Those forces act at right angles to the surfaces that they interact with, so the upward force and the downward force are equal and opposite for a "normal" scale that isn't hermetically sealed. Mar 13, 2019 at 1:16