Final answer:
During the upward swing of a pendulum, the conservation of linear momentum cannot be used because the motion is driven by the conversion of potential energy to kinetic energy and vice versa.
Step-by-step explanation:
During the upward swing of a pendulum, you cannot use the conservation of linear momentum because the pendulum's motion is not a result of external forces acting on it. Instead, the motion is driven by the conversion of potential energy to kinetic energy and vice versa. As the pendulum swings upward, potential energy increases while kinetic energy decreases, and this energy exchange is what governs the motion of the pendulum.