题在水平桌面M上放置一块正方形薄木板abcd,在木板的正中央放置一个质量为m的木块,如图1所示.先以木板的ab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;接着以木板的ab边为轴,将木板向上缓慢转动,转动角度也为θ,在转动过程中木块没有滑动.求此时木块受到的摩擦力的大小. Put a square thin board abcd on the horizontal tabletop M, and place a wooden block of mass m in the middle of the board, as shown in Fig. 1. Take the ab side of the board as the axis, turn the board slowly upwards to make the board The angle between ab’s edge and the desktop is θ; then the board’s ab edge is taken as the axis, the wood board is slowly turned upwards, and the rotation angle is also θ. During the rotation process, the wood block does not slide. Find the friction force of the wood block at this moment. the size of.