Final answer:
To lift a 100 N load with a force of 20 N, a mechanical advantage of 5 is needed, which corresponds to using 5 pulleys in the system.
Step-by-step explanation:
To determine how many pulleys are needed to lift a 100 N load with a force of 20 N, we need to calculate the mechanical advantage (MA) that the pulley system needs to provide. The mechanical advantage of a pulley system is the ratio of the load force to the effort force. In this case, the MA is 100 N / 20 N = 5. Therefore, each pulley provides a mechanical advantage of approximately 1 when we consider the effort required to pull the cable, thus effectively doubling the force applied to the system. To achieve a mechanical advantage of 5, 5 pulleys are needed, since we assume that the pulleys are frictionless and the system is an ideal machine, as depicted in Figure 9.27 where three pulleys provided a mechanical advantage of approximately 3. The correct answer is C. 5.