如图所示，有一质量不计的长木板，左端可绕O点转动，在它的右端放一重为G的物块，并用一竖直向上的力F拉着当物块向左匀速运动时，木板始终在水平位置保持静止，在此过程中，拉力F(    )

     What is color? Why do some of the things around us look red, some green, others blue?
    Colors are really made by deflected (反射) light. We see color because most of the things reflect light. In the same way, if something is green, it reflects most of the green light. If something reflects all light, it is white. If it doesn't reflect any light, it is black.
Some of the light is reflected and some is taken in (吸引) and turned into (变成) heat (热能).The darker the color is, the less light is reflected, the more light is taken in. So dark-colored clothes are warmer in the sun than light-colored clothes.