The term \"impact force\" has no meaning in physics. Impacts take a finite amoun

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The term "impact force" has no meaning in physics. Impacts take a finite amount of time during which some energy and momentum is exchanged. The force typically varies during this time - so there is no one "impact force". In a collision, a force-time graph looks like in inverted parabola. A totally immobile and rigid ideal post would have an impact duration of zero to produce a finite momentum change - which would give an infinite force. So that's probably not a useful model. Usually impacts are often investigated using accelerometers. You can characterize how hard the impact was using the specific impulse - which is the area under the first bounce of the f-t graph. You could also put a standard post-head (you don't need the whole post) on top of a heavy spring, strike the post and see how far the spring compresses - maybe against a ratchet. You can easily find out how the spring behaves under known blows and so create a scale to compare the new systems with. So if the vorticity of a fluid = 0, it is in steady state laminar flow and friction is negligible (and viscosity too?), you can use the bernoulli equation between any two arbitrary points in the fluid, regardless if they are connected by a streamline. If the vorticity is non-zero, can you still use bernoulli, but only if you follow a streamline?

Explanation / Answer

And how does the viscosity of a fluid come into this? I believe the viscosity must be negligible for the Bernoulli-equation to be used. However, without viscosity you wouldn't even have laminar flow, so what's up with that?

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Bernoulli's equation says nothing about laminar versus turbulent flow because it applies only to inviscid flows, and the concept of laminar and turbulent flow are meaningless. Bernoulli's equation requires that the flow be steady, inviscid and incompressible to be valid and applies generally to flows along a streamline. If the flow is also irrotational, then the Bernoulli equation is no longer restricted to a streamline and applies to the whole flow.

Of course, all fluids are viscous, so you might ask when you can ever use Bernoulli's equation. One consequence of boundary-layer theory is that outside of boundary layers, flows behave as if they were inviscid, so outside of the boundary layer, you can use Bernoulli''s equation to your heart's content. Inside the boundary layer, though, you can't, as viscosity is a major factor in those flows.

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