FAA Aviation Mechanic General (AMG)Physics, Math and Fluid LinesMedium
A mechanic uses a simple balance beam scale to weigh an aircraft component. A 100-pound reference weight is placed 20 inches from the fulcrum. To balance the scale, an unknown weight must be placed 25 inches from the fulcrum on the opposite side. What is the unknown weight?
- A80.0 lb
- B100.0 lb
- C62.5 lb
- D125.0 lb
Show answer & explanationAnswer & explanation
Correct answer: A. 80.0 lb
For a balanced lever, moments are equal: W1 × D1 = W2 × D2. So 100 × 20 = W2 × 25, giving W2 = 2000 / 25 = 80 lb.
Why the other options are wrong
- B. This assumes the weights must be equal, ignoring differing arm lengths.
- C. This results from dividing 100 by an incorrect ratio setup.
- D. This results from inverting the ratio instead of solving correctly.
Moment Balance (Torque Equilibrium)
A lever or scale is balanced when the moments (weight × distance) on both sides of the fulcrum are equal.
- Moment = Force (or weight) × Distance from fulcrum
- Balance condition: W1 × D1 = W2 × D2
- Used in aircraft weight and balance calculations
Memory trick: 'Equal moments keep the see-saw level' — weight times distance must match.