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- When calculating normal forces of, e.g., a block sliding down an incline (with friction), we disregard the weight of the air on top of the block. Why?

I once was explained the answer to this question - something about both the air and the object being incompressible. However, for the life of me I cannot seem to reproduce the answer of why we ignore air pressure when doing basic calculations.

If it matters, the particular system I'm interested in is that of a submerged particle in a fluid. Does the frictional coefficient of the particle to the surface depend on the amount of water above it? My intuition tells me two things:

a) that a particle that is even

b) the frictional coefficient is not dependent on the amount of fluid above it (pushing a penny at the bottom of a pool and at the bottom of a beaker requires an identical amount of work).

If it matters, the particular system I'm interested in is that of a submerged particle in a fluid. Does the frictional coefficient of the particle to the surface depend on the amount of water above it? My intuition tells me two things:

a) that a particle that is even

*slightly*more positively buoyant (with respect to the fluid) will float to the top regardless of how much fluid is on top of it, andb) the frictional coefficient is not dependent on the amount of fluid above it (pushing a penny at the bottom of a pool and at the bottom of a beaker requires an identical amount of work).