题目在水平桌面M上放置一块正方形薄木板abcd,在木板的正中点放置一个质量为m的木板,如图1所示,先以木板的ad边为轴,将木板向上缓慢转动,使木板的ab边与桌面的夹角为θ,再接着以木板的ab边为轴,将木板向上缓慢转动,使木板的ad边与桌面的夹角也为θ(ab边与桌面的夹角θ不变).在转动过程中木块在木板上没有滑动,则转动以后木块受到的摩擦力的大小为 Title Place a square thin board abcd on the horizontal desktop M, Place a m-quality board at the midpoint of the board, as shown in Figure 1, first with the ad edge of the board as the axis, slowly turn the board upwards, ab side of the angle between the desktop and theta, and then followed by the ab side of the wood as the axis, the wood slowly upward rotation, so that the angle between the ad edge of the wood and the desktop is θ (ab edge and the angle between the table θ unchanged ) During the rotation of wood in the wood without sliding, then the size of the friction by the wood after rotation