Problem 1.7[Difficulty: 3]Given:Small particle accelerating from rest in a fluid. Net weight is W, resisting force FD= kV, where V is speed. Find:Time required to reach 95 percent of terminal speed, Vt. Solution:Consider the particle to be a system. Apply Newton's second law. Basic equation: ∑Fy= mayAssumptions: 1. W is net weight 2. Resisting force acts opposite to V Then Fyy∑=−==WkV = madtmdVWgdVdtor dVdtg(1kWV)=−Separating variables, dV
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