For a lever, which ratio defines the mechanical advantage?

• A. Effort arm divided by resistance arm
• B. Resistance arm divided by effort arm
• C. Effort arm plus resistance arm
• D. Effort arm / Resistance arm

Answer: (D)

In a lever, how much you multiply your input force depends on the distances from the fulcrum. The lever balances torques: effort force times its arm equals resistance force times its arm. So F_in × d_in = F_out × d_out. From this, the mechanical advantage becomes MA = F_out / F_in = d_in / d_out. Put differently, it’s the length of the effort arm divided by the length of the resistance arm. When the effort arm is longer, MA is greater than 1, meaning you can lift a heavier load with less input force (though you have to move the effort farther). The correct description is the ratio of the effort arm to the resistance arm.