2.90) An isolated cylinder-piston set that contains a fluid has a stirring device moved from the outside. There is no friction in the plunger and the force holding it against the fluid is due to standard pressure and a coil spring. The spring constant is 7,200 N/m. The depression device is rotated 100 rev with an average torque of 0.68 N.m. As a result, the 0.10 m diameter piston moves 0.10 m outward. Calculate the change in internal energy of the fluid in kilojoules. If the initial spring force is zero.