How is the moment for a given weight and its location determined?

• A. Moment = weight × arm
• B. Moment = weight × distance from datum
• C. Moment = weight ÷ arm
• D. Moment = arm − weight

Answer: (A)

Moment is the turning effect produced by a weight about a reference point. For weight placed somewhere in the airplane, the moment about the datum is found by multiplying the weight (a force) by how far that weight’s line of action is from the datum—the lever arm. That distance is defined as the arm. So the moment equals weight times arm. This directly expresses the rotational effect: a larger weight or a longer arm increases the moment, while a shorter arm or lighter weight reduces it. In practice, you’ll see moments added together to get total moment, and then total weight and CG are found from those totals. For example, 2000 lb located 6 ft from the datum produces a moment of 12,000 ft-lb. The other forms don’t match how moment is defined or yield nonsensical units, so they aren’t used.