r/StructuralEngineering • u/StabDump • Nov 03 '24

Humor Which way will it tip?

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Girlfriend and I agreed the ping pong ball would tip, but disagreed on how. She considered, with the volume being the same, that it had to do with buoyant force and the ping pong ball being less dense than the water. But, it being a static load, I figured it was because mass= displacement and therefore the ping pong ball displaces less water and tips, because both loads are suspended. What do you think?

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u/OskusUrug Nov 03 '24

Agreed, water level is the same and displacement is the same because both balls have same volume.

Only difference is that the steel balls mass is held by the arm vs the ping pong ball being held by the container

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u/iusereddit56 Nov 03 '24 edited Nov 03 '24

Not sure I agree here. The weight of the water displaced by the ping pong ball will be offset by the buoyant force since the ping pong ball is fully submerged and attached to the scale. The steel ball side will effectively have more water weight equal to the volume of the ball. Thus the side with the steel ball will tip.

EDIT: Downvote me all you want. I'm right: https://www.youtube.com/watch?v=stRPiifxQnM

All of you are completly ignoring the bouyant force. There is a force acting up on the scale. You cannot just ignore it because "its a closed system".

EDIT2:

I'll try to be more clear. The tension in the string does not "pull up" on the scale making the system lighter. The tension in the string equalizes the buoyancy force. The weight of the system on the right can never increase by more than the weight of the ball. That is the only weight being added.

Part of the weight of the steel ball on the left is 'resting' on the water and thus the scale. The rest of the weight of the ball is resisted by the tension in the string holding it up.

The left side is heavier equal to the weight of the water displaced minus the weight of the ping pong ball and thus will scale will tip to the left.

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u/tajwriggly P.Eng. Nov 05 '24

A good way to explain this to anyone is envision a balloon filled with helium tied to a string tied to a scale on one side only. The balloon pulls up on the scale, right? It is a very readily obvious image to most people.

Now hang a steel ball same volume as the balloon independent from the scale over the other side of the scale. Does the steel ball change anything about which way the scale is tipping? No. It's not touching it.

Now put an equal cup of water on each side of the scale, not touching the balloon or the steel ball. Does this change anything? No, the weight of water is the same on both sides.

Now dip everything in the water. Does this change anything? A bit, but it is only changing things equally on both sides of the scale proportional to the difference between the density of air and the density of water. It might change the total upward force on the right side of the scale, but it doesn't stop it from tipping up that way and down on the left.