I am currently studying IGCSE Physics, and have the following question: Why aren't upthrust and normal reaction force not the same.
I understand that upthrust only occurs in fluids, and normal reaction forces only occur on a surface, but what is the actual, theoretical difference?
Best Answer
At the beginning I found this question a bit naive. But now I think it is worth to think a little bit about it.
They definitely share some properties, at least at the microscopic level. Both have the same microscopic origin: the electromagnetic interaction. The upthrust requires gravity to create a pressure gradient on the fluid, resulting in an upwards net force. The normal force that a surface does on a body resting on it also has gravity as a "trigger", though this is not always required. If someone presses the body against the surface then the correspondent parcel of the normal force has nothing to do with gravity.
From the macroscopic point of view they are quite different. The upthrust has always the direction of the pressure gradient. The direction of the normal force can be any, defined by the normal to the surface. The upthrust on a (completely immersed) body of volume $V$ depends only on its volume and the density of the agent (fluid). The normal force acting on the same body when resting over a surface depend on its mass instead and does not depend on physical properties of the agent (surfaces).
I think this is enough to see differences and similarities.