Final answer:
The internal energy of a molecule constrained to translational motion is its translational kinetic energy which is the sum of the motion of all the molecules in the system.
Step-by-step explanation:
The internal energy of a molecule that can only move in a translational manner is its translational kinetic energy. In a monatomic ideal gas the only form of energy that the molecules possess is their translational kinetic energy. Therefore the internal energy of such a system is equivalent to the sum of the kinetic energies of all the molecules moving in translation.
At very low temperatures such as below 60 K for diatomic gases, translational kinetic energy is the only form of energy due to restrictions by quantum mechanics. However at higher temperatures other forms of energy such as rotational and vibrational can become active and contribute to the internal energy.