How should the 500-pound weight be shifted to balance the plank on the fulcrum?

• A. 1 inch to the left
• B. 1 inch to the right
• C. 2 inches to the left
• D. 2 inches to the right

Answer: (A)

Balancing a plank on a fulcrum depends on equalizing moments (torques) around that pivot. A weight on one side creates a torque equal to its weight times its distance from the fulcrum. To bring the net torque to zero, you adjust the distance of the weight from the fulcrum. Shifting the 500-pound weight toward the fulcrum by one inch reduces its moment arm, and thus its torque, enough to balance any opposing torque on the other side. If you moved it farther away, the torque on that side would increase and the plank would tilt in that direction. So moving it one inch toward the fulcrum (to the left) is the correct action.