Question 1
The total force needed to drag a box at constant speed across a surface with coefficient of kinetic friction μk is least when the force is applied at an angle θ such that
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sinθ = μk.
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cosθ = μk.
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tanθ = μk.
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cotθ = μk.
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secθ = μk.
Question 2
The vector sum of three co-planar forces
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must be zero.
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must be perpendicular to one of the three.
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must be parallel to one of the three.
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must be perpendicular to the plane.
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may have any direction in the plane.