while one end of the board is being raised,what is the effec
while one end of the board is being raised,what is the effect on the force of friction between the board and the object?
答案给的是steadily increases为啥呢?
回复一楼的:没有图,它属于前几题的
不好意思补充下:前面题设还说是转到50°就停了.也就是说,三楼的细致地讲下为什么增大就好了