Final answer:
The person must lean forward or slide their feet under the chair to stand up in order to maintain equilibrium by aligning their center of gravity with their base of support, making the process of standing up more stable and requiring less energy.
Step-by-step explanation:
The reason why a person must either lean forward or slide their feet under the chair in order to stand up is to keep the body in equilibrium while rising. When standing up from a seated position, it is crucial to maintain one's center of gravity above the base of support provided by the feet. Leaning forward or moving the feet under the center of gravity helps achieve this balance. The center of gravity is a point where the total weight of the body is concentrated, and managing this point is necessary to prevent falling.
As a chair-bound person leans forward, their center of gravity shifts above their feet, allowing them to stand. Similarly, sliding the feet under the chair brings the base of support closer to the center of gravity. This reduces the moment arm, the distance between the base of support and the center of gravity, which makes standing up easier and more stable. This technique also minimizes the use of extensive muscle power, thus indirectly reducing the energy required to stand up, but the primary goal is to maintain equilibrium.