Take into account that if the system is in equilibrium, then, the sum of torques must be equal to zero. In this case, both weight of the block and applied force F generate a torque over the central fixed point with different signs:
W is the weight of the block, L1 the distance fromthe block to the pivot, F the applied force and L2 the distance between the point of the application of the force and the pivot.
The previous equation can be written as follow:
The previous relation determines the equilibrium of the system.
Then, you can conclude:
(i) If F is increased, then, the board will rotate clockwise, because the torque produced by F will be greatest than the torque produced by the weight.
(ii) If is decreased, then, the board will rotate counter-clockwise, because the torque priduced by the weight will be greater.
(iii) If the applied force approaches to zero, then, the board will rotate counter-clockwise again, because there will be no torque at the right of the pivot.
(a) Now, if the weight is moved closer to the pivot (pin), the torque at the left appraches to zero, and then the board will rotate clockwise.
(b) If the applied force is moved to a position closer to the pivot, the torque at the right approaches to zero, and then, the board will rotate counter-clockwise.
(c) If the weight of the block decreases to 50N, it means that the torque at the left will decrease to ione half of its initial value. Under this new condition the equilibrium of the system breaks down and the board will rotate clockwise.