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For this problem on the topic of equilibrium and elasticity, we are shown a block of mass 10kg, which is an equilibrium.
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This block labeled block a is on a surface that has friction.
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It would slip if block b were to weigh any heavier.
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Block b's mass is 5kg.
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The angle that is given theta is 30 degrees, and we want to find the coefficient of static friction between block a and the surface below block a.
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So we can draw free body diagrams for blocks a, b, and c, or the knot rather, which is denoted as c, as we've drawn here.
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And the tensions in the three strings are t, a, tb, and tc.
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So if we analyze forces at c, since we have static equilibrium, the conditions for static equilibrium tell us that tc times the cosine of theta must equal...