In order to explain the change in energy of an ice cube removed from a refrigerator and placed on the kitchen counter, I consider it as a closed system (can exchange only energy but not matter). For the first law of thermodynamics, ∆U = Q+W. I'm pretty sure that the sign of Q is positive due to the temperature difference that results in the energy transfer. What I'm not sure about is the work. In the process that ice melts, I figure there might be a increase in gravitational potential energy because its center of gravity descends, so should the sign of W be positive, too? I'm also not sure about the nature of those three types of energy. I guess the nature of internal energy is the microscopic motion of the particles in the system, but what about the other two? Any help would be appreciated!