Resumen de Spin Direction of a Hemispherical Object
Edy Wibowo, Mamat Rokhmat, Naily Ulya, Sutisna *
When rotating a ball using a hand [Figs. 1(a) and (b)], the fingers that touch the ball’s outer surface will exert frictional forces on the ball. These frictional forces cause torques on the ball so that the ball spins in the same direction as the finger’s rotational direction. If the finger rotates clockwise, the ball’s rotational direction will also be clockwise and vice versa. When the ball is cut into half, it is rotated similarly to when spinning a full ball. Thus, we can easily guess that its direction of rotation will be the same as when circling a full ball. What if the ball is spun differently? The half ball is spun using one fingertip from the bottom center of the inside, as shown in Figs. 1(c) and (d). What is the direction of rotation of the half ball? Would it be the same as when the ball is rotated from the outside? It turns out that the spin direction of the half ball is the opposite of the finger’s spin direction. The same result was observed when using a pen or a stick to rotate the half ball from the center base [Fig. 1(d) and supplemental video. Finally, suppose we shift the finger’s tip to any location so that it will no longer be at the center of the bottom of the object. In that case, the half ball’s rotation direction is similar to the one of the finger or stick. Why does this happen? In this paper, the answer to this question is presented.
