Each elements emission spectrum is distinct because each element has a different set of electron energy levels. The emission lines correspond to the differences between various pairs of the many energy levels. The lines (photons) are emitted as electrons fall from higher energy orbitals to lower energies.
In the vertical direction, if the friction force (directed upward) is equal to the force of gravity (downward) this means that no net force is acting on the firefighter.
According to Newton's 2nd Law, if no net force is present, the acceleration in this direction is just zero, as follows:
