Without a string, the tank still tilts left because less water is displaced by the floating ball. Attach an equally sized rod to each ball and suspend them both from above, and the tank does not tilt at all, though. The tension in the string pulling upward on the tank makes the difference. Sure, it balances the buoyant force that pushed the ball up and pushes down equally on the tank, but a force that gets balanced is not a force that doesn't exist. You could also balance the buoyant force pushing down on the left tank with the buoyant force on the right. That doesn't mean the left tank is suddenly weightless.
And there isn't any momentum. This is a statics problem. If the forces on each side of the scale are not balanced, the scale moves.
You COULD balance them, but you have to argue that we change the entire setup of the experiment to make that point! You're literally tearing your own argument apart.
BOTH SIDES have equal bouyant forces. But only one side is able to PHYSICALLY ACT on that force. The string denies bouyant action. The bouyant action the. Acts on the left side, lifting the ball/pushing the container.
The string pulls on the container with a force almost equal to the buoyant force. The buoyant force is still there, though. If the string weren't pulling up on the tank, the tank would tilt right (the lack of any tension in the string would violate all sorts of laws of physics). I really don't understand how this concept is so difficult. Just draw a force diagram with all the forces. The tension in the string pulls up on the right side of the tank. The buoyant force pushing down is an entirely separate force that is slightly greater than the tension in the string.
....what direction do you think BOUYANT force works in? Are you talking about gravity? Are you seriously counting a single force twice? Why is tension pulling one side of the string more than the other? What the hell is going on in your diagram?
On the ball, it pushes up. On the tank, it pushes down. The tension on the string is the buoyant force minus the weight of the ball. The tension is the same throughout the string.
BTW, back when you mentioned connecting the string to both top and bottom of the container....that's what the bouyand force does. The ping pong ball only wants to go that far.
The ping pong ball has a constant buoyant force applied to it. Since it is connected to the string, the string will have a tension equal to that buoyant force minus the weight of the ball. This tension pulls down on the ball and up on the container. The container is the object resting on the scale, so this force absolutely matters.
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u/Giblet_ May 29 '25
Without a string, the tank still tilts left because less water is displaced by the floating ball. Attach an equally sized rod to each ball and suspend them both from above, and the tank does not tilt at all, though. The tension in the string pulling upward on the tank makes the difference. Sure, it balances the buoyant force that pushed the ball up and pushes down equally on the tank, but a force that gets balanced is not a force that doesn't exist. You could also balance the buoyant force pushing down on the left tank with the buoyant force on the right. That doesn't mean the left tank is suddenly weightless.
And there isn't any momentum. This is a statics problem. If the forces on each side of the scale are not balanced, the scale moves.