We present a simple model to describe evaporative cooling of ultracold trapped atoms. The cooling is used to achieve low temperatures of the gaseous sample by removing the most energetic atoms and subsequently rethermalizing the remaining atoms at a lower temperature. The model assumes a Maxwell-Boltzmann distribution of energies and allows the calculation of the temperature and the number of remaining atoms after the most energetic atoms have been removed and rethermalization has taken place.

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