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Unformatted text preview: 1000 lb weight is only about
4.4 × 103 N. In other words, even with a 5 cm sealing surface, there is no way you’re going to open
that door at altitude while the cabin is pressurized.
2. Viscosity of most ﬂuids can be represented by an extra “drag” force on a body moving in a liquid.
For a body of spherical shape, the drag force is reasonably well approximated by Fdrag = 6πηRv,
where v is the velocity of the body and η is a parameter of the ﬂuid. The presence of viscosity
leads to a “terminal velocity” of a body falling in a ﬂuid (e.g., a person falling in air).
Consider a sphere of radius R and density ρs falling through a ﬂuid of density ρ and viscosity
parameter η. Find an expression for the terminal velocity of the sphere.
Solution: Terminal velocity is when the the object in question reaches a constant maximum
velocity, which must be when the net force on the object is zero. Physically, the object’s speed has
become so high, and the corresponding drag force s...
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