Equilibrium of a Rigid Body under Coplanar Forces

Applied Mathematics — Second Secondary

Common mistakes

Comparison aspect	Main idea	Keywords
Mistake	writing the condition of equilibrium as “the sum of the forces = 0” without resolving them which means adding magnitudes of forces with different directions. Correct: add the horizontal and vertical components separately	writingconditionequilibriumforces
Mistake	leaving out the weight or the normal reaction from the force diagram. Correct: draw all the forces acting on the body before writing any equation	leavingweightnormalreaction
Mistake	forgetting the negative sign of a component that points in the negative direction of its axis. Correct: give every component pointing in the negative direction of its axis a negative sign	forgettingnegativesigncomponent
Mistake	using tan θ for the angle of a force without checking its quadrant. Correct: check the signs of the two components to find the quadrant	usingangleforcewithout

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