Static Equilibrium

Engineers need to design and build structures which don't fall over, slide, or tilt. In other words, they need to make sure their buildings remain in static equilibrium: no translation, no rotation.
An object is in static equilibrium if and only if:
- The sum of the forces on it in each direction is zero.
- The sum of the torques on it in each direction is zero.
- Its linear momentum is zero (i.e. it's not moving).
If the forces on an object do add up to zero in each direction, then one can pick any axis around which to calculate the torques. It usually helps to pick an axis which is exactly where one force is applied -- that will make the torque due to that force zero.
Margins of safety are Good.

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