f？ Lcosθ=MgL2cosθ+mglcosθ
Where: L is the length of the board, and L is the distance between the block and the supporting point.
Solution: f = 12 mg+llmg, which has nothing to do with angle θ, so A is wrong;
B, what is the torque of the pulling force F? Lcosθ, so it's getting smaller and smaller. Therefore, B is correct.
C. The wood block M is subjected to gravity, supporting force and static friction, in which the supporting force and static friction force are the forces exerted by the board on the slider. According to the balance condition of three forces, the supporting force and static friction force must be equal to gravity. According to Newton's third law, the force of a block on a board is also equal to the gravity of the block, so C is correct;
The stress analysis of B and M shows that M is subjected to gravity, supporting force and static friction, as shown in the figure.
According to * * * point force balance conditions, there are
f-mgsinθ=0
N-mgcosθ=0
solve
f=mgsinθ
That is, the static friction increases with the increase of θ, so d is wrong.
So choose BC.