3: Notes

Particle Equilibrium​

For a particle to be in equilibrium, all forces acting on the particle form a zero resultant force
It is necessary to draw a free body diagram to account for all the forces that act on a particle
F_R = ΣF = 0

2D​

The two scalar equations of force equilibrium can be applied with reference to an established x, y coordinate system

ΣF_x = 0
ΣF_y = 0​

TENSION & pulleys - The tension force in a continuous cable that passes over a pulley is constant throughout the cable to keep it in equilibrium
SPRING - If the problem involves an elastic spring, then the stretch or compression (s) of the spring can be related to the force applied to it

F = ks​

3D​

First express each force on the free-body diagram as a Cartesian vector, then when the forces are summed and set equal to zero, the i j & k components are also zero. 

ΣF_NET = 0
ΣF_x = 0
ΣF_y = 0
ΣF_z = 0
​