A point PP is subjected to three simultaneous forces of magnitudes FA>FB>FC. Point PP is in equilibrium. Which of the following statements is not always true about the magnitudes of the forces?
|.$F_A=F_B+F_C$
||.$F_A\leq F_B+F_C$
|||.$F_A\gt F_B-F_C$
A. I only
B. I and II only
C. I and III only
D. I and III only
参考答案: A
本题详细解析: Vectors of forces in equilibrium form a triangle or a straight line. The sum of two sides of a triangle is always greater than the third (II is always true). The difference between two sides of a triangle is always less than the third (III is also always true). I is true only when the three forces are in a straight line and FB and FC are both opposing FA(not always true, as required by the question).
