A gas is enclosed in a system similar to that shown in Figure 18.2. More weights are added to the piston, giving a total mass of 2.20 kg. As a result, the gas is compressed and the weights are lowered 0.250 m. At the same time, 1.50 J of heat evolves from (leaves) the system. What is the change in internal energy of the system, ∆U? The force of gravity on a mass m is mg, where g is the constant acceleration of gravity (g = 9.80 m/s2).