let’s say I have spooned some tomato paste into a glass jar, and there are all these pockets of air and I want to collect the tomato paste at the bottom of the container.
The tomato paste is viscous / thick enough that gravity is not causing it to settle anytime soon, so if I pick up and then slam the jar down, the forces can cause the paste to push down and settle into the jar.
I notice if I slam the jar on a cutting board or a hard counter-top, the tomato paste doesn’t settle very much. When I slam the jar on a rag, it seems like it settles better.
If I had to guess, I’m curtailing some of the force when hitting hard surfaces because I’m worried about the glass breaking, and I don’t like the noise so I hold back to reduce noise.
A folded-over damp rag however is softer and cushions the jar, so it doesn’t make noise and it feels like I can get away with using more force - so maybe more force = better settling?
I’m tired, so I thought I would ask why the rag trick works - but now I think I understand.
Anyway, let me know if you disagree with my reasoning here, or if I’ve overlooked something.
Hard objects collide elastically which transfers more force in the opposite direction after the collision. Adding a rag, towel, or pillow softens the surface and the collision is now inelastic as some of the force of the jar colliding with the countertop is absorbed by the fibers of the cloth. Think what would happen if you dropped a glass marble on your countertop versus on a towel on your countertop. The marble would likely bounce up in the first scenario and not in the second. Similarly, the sauce in the jar experiences upward force from the jar after the elastic collision with the countertop but not with the rag softening the inelastic collision. You could optimize sauce flow to the bottom by changing the method used to apply forces to the jar. Get a string, tie it around the lid of the jar, and swing the jar around your head a few times. If you don’t yeet the jar against the wall accidentally, almost all of the paste/gel/viscous liquid should be at the bottom of the jar afterward.
Hard objects collide inelastically which transfers more force in the opposite direction after the collision. Adding a rag, towel, or pillow softens the surface and the collision is now elastic as some of the force of the jar colliding with the countertop is absorbed by the fibers of the cloth.
Isn’t this backward? Hard objects like billiard balls collide elastically, while soft objects like clay collide inelastically.
Totally right, I got this backwards. Fixed original comment.
Probably works in the same way as a dead-blow hammer, i.e. dampening rebound from the hard surface.
oooo, thank you - I didn’t know this was a thing, but that’s sorta how it feels … so what’s physically happening - the towel is able to absorb the force from the jar better? the harder surface makes the jar bounce off it more, so those rebound forces work against the gravity that settles the tomato paste?
Yep! That is my guess at least. I’m not a scientist, though …
I’m not a physicist, but I’d imagine the difference is between a sharp slow down and a more gradual one. When you’re using the wet rag, the point of impact is a few milliseconds longer than when rapping it on a hard surface.
hm, that’s an interesting theory - I do think I’m able to get a sharper impact with the wet rag, there is no slow down before hitting the surface whereas maybe I’m not being as forceful but I’m also slowing down the jar more before hitting the hard surface? I guess that’s not that different than just using less force, right?
I think it’s because a sharp blow will cause a single large wave front to propagate through the medium.
And with a dull blow, probably multiple smaller wave fronts that can interfere and create mini pressure differentials thus more settling
What you are describing is a non Newtonian fluid. Maybe your paste has additives?
hm, I don’t think so - non-Newtonian fluids are like water + cornstarch: they flow like a liquid unless you apply force, then they harden into a solid.
The tomato paste is just not a liquid, it’s so thick it functions like a solid. So if you spoon some of it into a jar, the paste will just stay where you spooned it, it won’t settle like a liquid would. AFAIK, a non-Newtonian fluid would settle like a liquid when not under force, unlike the tomato paste.
they flow like a liquid unless you apply force, then they harden into a solid.
Not all non-Newtonian fluids are like this. Some are reversed: They flow like a liquid when pressure is applied, and harden into a solid (or gel) when the pressure is released. I think ketchup is one such example.
