Final answer:
Increasing resistance force on a lever requires a larger effort force if the lever's arm length remains the same due to the balance of torques and the relationship between effort arm and resistance arm for mechanical advantage.
Step-by-step explanation:
When resistance force is increased on a lever, effort force must also increase if the lever arm remains constant. This relationship is due to the principle of levers which states that the effort force times the effort arm (the distance from the fulcrum to the point where the effort is applied) is equal to the resistance force times the resistance arm (the distance from the fulcrum to the point of resistance). The mechanical advantage of a lever is the ratio of these forces, and by increasing the resistance force, the lever's mechanical advantage decreases unless the effort arm is lengthened.
In a practical example, using a lever to lift a heavier stone (a larger resistance force) while keeping the lever arm constant will require a larger effort force to achieve balance and lift the stone. Efficiency of a lever also plays a role; as efficiency decreases due to factors like friction, even more effort may be necessary to overcome the resistance.