力的平行四边形法则 The parallelogram law of force
二力平衡条件为等大，反向，共线 The two-force balance condition is equal, reverse and collinear
在任一原有力系上加上或减去任意的平衡力系，与原力系对刚体的作用效果等效
Adding or subtracting any balance force system to or from any original force system is equivalent to the effect of the original force system on rigid bodies
作用力和反作用力总是同时存在，两力大小相等，方向相反，沿着同一条直线，分别作用在两个相互作用的物体上
The acting force and the reaction force always exist at the same time, the two forces are equal in magnitude, opposite in direction, and act on the two interacting objects along the same straight line.
变形体在某一力系作用下处于平衡，如将此变形体刚化为刚体，其平衡状态保持不变
The deformed body is in equilibrium under the action of a certain force system. If the deformed body is rigidized into a rigid body, its equilibrium state remains unchanged
