Final answer:
The portable ladder must extend 3 feet above the upper level of access to ensure safety. This applies to scenarios involving calculations of forces on a ladder in physics problems.
Step-by-step explanation:
The distance above the upper level of access in which a portable ladder must extend is 3 feet. This ensures that an individual has something to hold onto when stepping off the ladder onto the next level, improving safety and compliance with Occupational Safety and Health Administration (OSHA) requirements. When calculating forces involved with a ladder, one must consider the mass of both the ladder and the individual, the location of the center of mass, and the points where the ladder makes contact with the ground and the support surface.