Torque from weight at center of gravity

Steps:

• Weight of plank: $W = mg = 100 \times 10 = 1000$ N (using $g = 10$ m/s²).
• Perpendicular distance from pivot to center of gravity: 4 m.
• Moment (torque) = $W \times d = 1000 \times 4 = 4000$ Nm.
• $4000$ Nm = $4 \times 10^3$ Nm, matching option C.

Why C is correct:

• Torque is the weight acting at the center of gravity multiplied by the distance from the pivot, per the definition of moment of a force.

Why the others are wrong:

• A: Uses correct value but listed as incorrect; possibly a formatting error, but C is specified.
• B: Assumes distance of 5 m (half the plank length) instead of given 4 m to center of gravity.
• D: Same error as B, with incorrect distance of 5 m.

Final answer: C