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How would you determine whether an object is at equilibrium or not?

What is the definition of equilibrium?

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closed as too localized by ja72, Waffle's Crazy Peanut, Brandon Enright, Emilio Pisanty, twistor59 Jun 7 '13 at 16:13

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By comparing the object against the definition of equilibrium of course. Could you be more specific about what you're having trouble understanding? – David H May 1 '13 at 0:50
This question (v3) seems closable since a straightforward Google search would immediately provide the answer, cf. e.g. this and this meta post. – Qmechanic May 31 '13 at 7:55

Static equilibrium requires that the sum of all forces is 0,$$\sum F=0$$ and the sum of all torques is 0, $$\sum \tau =0$$ Be aware that force and torque is a vector; they have both magnitude and direction.

You will usually want to separate a force into its components such that all forces are either parallel or perpendicular to each other. Then apply the equation above for force separately for x- and y- axes.

The direction of torque follows the right-hand rule. If unsure, just think of counter-clockwise as positive and clockwise as negative. Then apply the equation for torque.

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There a several notions of equilibrium. These are discussed in an informative online encyclopedia called "Wikipedia", which I recommend. The link to the mechanical equilibrium section is here. (I assume you mean mechanical equilbrium since there is the forces tag.) If you read the article and have trouble understanding anything, please ask a more specific question about what you are having trouble understanding.

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