Final answer:
A girl on a trampoline has maximum gravitational potential energy at the highest point of bounce, and maximum kinetic energy at the lowest point when she contacts the trampoline.
Step-by-step explanation:
When a girl is bouncing on a trampoline, her gravitational potential energy is at a maximum when she reaches the highest point in her bounce. At this point, her kinetic energy is zero because her speed is momentarily zero. As she descends from this point, her potential energy decreases, and her kinetic energy increases, reaching a maximum when she makes contact with the trampoline, which is at its lowest point of compression during the bounce.
By the conservation of mechanical energy principle, the mechanical energy of the girl remains constant (ignoring air resistance), and the energy is just interchanged between potential and kinetic forms. When the gravitational potential energy is at its maximum, the kinetic energy is at its minimum (virtually zero), and vice versa.