Identify the sign of ΔU for each indicated system.
a) A gas sample is compressed adiabatically. (System = gas)
b) Hot coffee is cooled in a refrigerator. (System = coffee)
c) A worker pushes a trolley along a ramp (System = worker)

• A. a) ΔU_sys < 0; b) ΔU_sys > 0; c) ΔU_sys > 0
• B. a) ΔU_sys < 0; b) ΔU_sys < 0; c) ΔU_sys < 0
• C. a) ΔU_sys > 0; b) ΔU_sys > 0; c) ΔU_sys < 0
• D. a) ΔU_sys > 0; b) ΔU_sys < 0; c) ΔU_sys < 0

Answer: (D)