![]() It's just that usually when you're talking about friction (such as air resistance) you don't usually think about layers of fluid but instead simplify the force as proportional to an object's velocity, such as F = k v or F = k v^2.į = μ N is just an approximation and only applies to dry friction. It may still be hard to believe there is zero resistance, but even if there was another resistance force, by definition it is not the viscous force, but some other force we had assumed didn't exist. The "inertial drag" has nothing to do with velocity, only acceleration, and therefore is not viscosity. But still, you can't blame the mathematics.īesides, if the board and water are already moving, there sure is no resistance. So the water increases inertia, but doesn't provide a resistance force.Įxcept with a depth of infinity the mass would probably be infinity too, so it would be impossible to drag the water (a = F/m = F/∞ = 0). If the board is 1 kg, the water is 2 kg, and the force pushing the board is 6 N, then acceleration is not 6 N / 1 kg = 6 m/s^2, as is the case when there is no water, but 6 N / (1 kg + 2 kg) = 2 m/s^2, with a = F/m. However, by dragging the water with us we need to conquer its inertia. There is no force preventing that because there is no "ocean floor" to provide the friction, and we are assuming there is no other resistance. I think if the depth were really infinity, there would be no friction, because we could perfectly well drag all that water with us.
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