막대자를 둘째 손가락으로 수평으로 들어서 손가락을 동시에 부드럽게 움직여라.(그림 2.22) 막대자가 손가락 위를 부드럽게 미끄러질까? 아닐것이다.
한 손가락 위에서 먼저 미끄러질 것이다.
왜 손가락이 번갈아 가며 미끄러질까?
The finger that is first moved slides beneath the stick with a kinetic coefficient of friction.
The stick does not slide over the other finger because the static coefficient of friction there is larger.
The magnitude of friction on either finger depends not only on the friction coefficient, but also on the weight of the stick on the finger. As the moving finger is brought toward the center, more and more of the stick's weight is on that finger.
Eventually the friction on the finger is greater than on the other finger in spite of the difference in friction coefficients. Then the first finger stops and the other finger begins to slide, Such an exchange of motion can occur several times before both fingers are at the center.
