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I wish to explain the concept of the buoyant force without using the concept of pressure. Particularly, I want to explain why some objects float in water while some others sink, but I am finding it a lot more challenging that I initially expected. While I am thinking about trying to talk about the forces that the water molecules apply on the object, I have trouble relating this to the density of the water.

Does anyone have a good explanation or analogy that can be easy to understand for someone who has no physical intuition of physics?

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    $\begingroup$ "I am thinking about trying to talk about the forces that the molecules apply on the object. " forces acting over the entire area of the object in contact with the liquid? Sounds a lot like pressure $\endgroup$ Mar 13, 2021 at 19:08
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    $\begingroup$ You may try to use a more microscopic entity than pressure, for example, the microscopic stress tensor. But at the end of the day, after performing an average, you'll end up exactly with the gradient of the pressure as the cause of buoyancy. In no case I would call such an approach as intuitive. $\endgroup$ Mar 13, 2021 at 19:10
  • $\begingroup$ What age group are you trying to teach the concept to? Could you hopefully introduce the concept of pressure by having students blow up a balloon? The concept of buoyant force will be VERY difficult to teach without using pressure in the explanation. $\endgroup$ Mar 13, 2021 at 20:33
  • $\begingroup$ I'm going to do a small talk to a group of adults who are unfamiliar with physics, which is why they wont really have a scientific understanding of what pressure is $\endgroup$ Mar 13, 2021 at 20:38
  • $\begingroup$ Perhaps your first hurdle is to convince your pupils that a liquid can push upwards. An old wellington boot filled with water, with a hole punched in the upper might do the trick! $\endgroup$ Mar 13, 2021 at 22:30

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Sounds as though you need Archimedes' Principle

'The upthrust on a body partially or completely immersed in a liquid is equal to the weight of the liquid displaced'

Which means if a ball of volume $0.2m^3$ is under water, it'll displace $0.2m^3$ of water and get an upthrust equal to the weight of that water. If the ball is floating and half of it is underwater, it displaces half the volume of water (so the weight of displaced water is halved) and receives half the previous upthrust.

As you might know Archimedes shouted 'Eureka' when realising how to measure the volume of a crown, by using displaced water, whilst having a bath - and ran down the street naked!

https://en.wikipedia.org/wiki/Archimedes%27_principle

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  • $\begingroup$ P.S. if you are wondering about the term displaced water, you can fill a pan with water then lower an object into it, so it floats or sinks and look at how much water flows over the sides of the pan - the 'displaced water'. There are specially made 'displacement cans' made to demonstrate this e.g. in schools, which can direct the water into a measuring cylinder. $\endgroup$ Mar 14, 2021 at 10:29
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As the weight of the an object pushes on the surface of some water, the molecules of water push back against the object but can only stop the object from sinking if they can create a force equal to the weight of the object. To do this the water molecules must have the same or greater density than the object. If their density is less than the density of the object then the object sinks to the bottom. It sinks faster the greater the difference between its density and the water' s density because the net force on it is larger.

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  • $\begingroup$ This does not address the question. $\endgroup$
    – my2cts
    Mar 13, 2021 at 23:26

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