✔ 最佳答案
I suppose you count the elements from left to right, i.e. the left most element is 1, the one to its right is element 2... etc.
In the lower diagram (Fig b), element 3 is NOT at the equilibrium position. Instead, element 3 reaches the maximum displacement to the right from its equilibrium position.
If you plot a displacement-distance graph on Fig b, you will get a complete cycle of a sine curve, with elements 1 to 5 constitute the +ve half of the sine curve and elements 5 to 9 constitute the -ve half of the sine curve. Element 3, in fact, is located at the crest of the sine curve.
Element 7 (i.e. the 3rd element when counted from the right) behaves similarly as element 3, except that it is displaced to the left. Hence, this element is located at the trough of the displacement-distance sine curve.
