Deyang patent excavator

Solution: A, the stress analysis of the stone shows that the stone must bear gravity, the support force N of the cliff and the support force F of the excavator, as shown in the figure, when Fsinθ=mg, the stone is not subjected to static friction, and when Fsinθ≠mg, the stone is subjected to static friction. So the force on the stone at this time may be three or four. Therefore, A is correct.

B, if the direction of the static friction of the stone is upward, as shown in the figure, the greater the diagonal support of the excavator, the smaller the friction of the cliff to the stone. So, B is wrong.

C. If the supporting force of the excavator is appropriately reduced, the direction remains unchanged, the maximum static friction of the stone is reduced, and the gravity of the stone remains unchanged, then the stone can be slowly lowered. So, C is correct.

D, the direction of the excavator support force increases gradually, and the component of the support force along the cliff is Fsinθ, but the maximum static friction of the stone also increases, which may not make the stone move upward along the cliff. So, d is wrong.

So choose AC.