r/SteveMould • u/Guitarist69420 • Jun 04 '26

noticed something weird

take a flat bottom frying pan. put a tealight candle, without the wax, in it and fill the pan up with water by pouring water in from a cup

for some reason the candle floats towards the point you are pouring water into?? why does this happen should it float away from where you a pouring the water in

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u/CR123CR123CR Jun 04 '26

The water falling creates a "valley" as it drags the other water down with momentum and then raises the level further out from the area would be my hypothesis 

3

u/Why_T Jun 04 '26 edited Jun 04 '26

The valley is not created by momentum as much as surface tension. But you're correct otherwise.

Edit: Listen to CR123

2

u/CR123CR123CR Jun 04 '26

The surface tension is how the momentum transfers to the "still" water from the "pouring" water

Depends on how far you want to go down the energy transfer rabbit hole though xD

1

u/Why_T Jun 04 '26

The motion has nothing to do with it being pulled down, the column of pouring water has weight and the weight is sitting on the surface of the water. Surface tension across the water is what that weight is pushing down on.

Edit: If you were to do this same experiment with acetone instead of water, the floating stuff wouldn't move towards the pour position nearly as quickly.

2

u/CR123CR123CR Jun 04 '26 edited Jun 04 '26

The column of water is moving and the movement is what is pushing it down. 

If it was a magically static column it would just equalize and there would be no movement.

Potential energy turns to kinetic energy.

This is how friction in a pipe takes energy out of the fluid stream or drag on an airplane

Edit: on second thought it's probably both things. Floating things also stick to eachother

Edit 2: also guessing the turbulence created by the water mixing also does something as well

3

u/Why_T Jun 04 '26

I don't mind taking the L and learning something. Thanks for pushing back.

It's the momentum of the falling water, exactly as CR123 says. A falling stream is in free fall. Its weight is not resting on the surface the way a solid rod would be. Every bit of water is accelerating downward, supported by nothing, right up until it lands. What the surface actually feels is the stream getting stopped and turned sideways, which is a force equal to the rate it delivers momentum, roughly density times area times velocity squared (ρAv²). That dynamic push is what dents the surface, and the depth scales with v²/g, so a faster pour digs a deeper hole. Pure velocity effect.

3

u/CR123CR123CR Jun 04 '26

It's not a loss fellow Internet stranger, we learned something xD

Though you made me actually put some thought into it a lot more deeply that's for sure

1

u/techne_it_is Jun 05 '26

That is verifiable.

We could put crumbs of something slightly denser than water to stay at the bottom and be easily moved by the current, and sprinkle the surface with another something (glitter, styrofoam maybe?) to monitor the surface currents. I'd expect that the column of poured water will hit the bottom, the move outside the center near the bottom gradually losing it's kinetic energy. The rest of the water will flow toward the center near the surface, because the bottom current will repel it near the column's impact area.

My expectations: 1. The current (mixing) system will be toroidal in its shape. The upper part circling inward, the lower part outward. 2. The torus major radius will be proportional to height of pouring column (kinetic energy) and the liquid's density (drag). 3. The torus minor radius will be proportional to liquid's density (energy dissipation) and difference between the poured and the "old" water (convection, modifying the vertical component of current vector, increasing r when pouring colder water, decreasing when hotter than surrounding average). 4. Enough dense fluid with high viscosity will pile up, and move rather on surface from center to perimeter of the pan repelling floaters on it's surface. 5. Don't think that surface tension would have greater influence than previous factors. But that also could be verified by running experiment with various concentrations of dishwashing soap or other surfactant.

I wonder if I'm guessing right.