how can internal energy do work? how can internal energy do work? As far as i know, internal energy is the sum of KE and PE. But how can this, in turn, do work? It would be great if you can give me some scenarios. Thank you. 
 A: Take the example of system consisting of ideal gas and an insulated piston/cylinder so that no heat transfers occur.
The gas molecules have kinetic energy and are constantly impacting the piston. If the gas expands against the piston (displaces the piston) the gas does work (force times distance). The work is done at the expense of some of the kinetic energy of the gas molecules, that is, at the expense of some of the internal energy of the gas. Since the temperature of an ideal gas is a measure of the average translational kinetic energy of the gas molecules, the temperature decreases.
Bottom line, the work done by the gas equals the decrease in the internal energy of the gas. 
Hope this  helps.
A: Internal energy arises from the energy of all its parts, or the atoms/molecules/electrons/whatever that compose it. If these particles have lots of energy on average, they can transfer this energy to something else. 
For example, a hot gas has a large internal energy since the atoms have higher kinetic energies and velocities. If you put this hot gas in a balloon, it will expand the balloon since the high energy atoms will collide with the boundaries. In other words, the gas does work on the balloon by transferring the kinetic energy of its atoms (which all contribute to the internal energy).
