HELP PLEESSS What is the definition of net force?

Oxygen undergoes an isentropic expansion from 120 degree c and 280 kpa absolute to 140 kpa absolute. determine the final temperature of the oxygen and the amount of work per kg of o_2 produced in the process if it is adiabatic.

Una carga puntual de 3 x 10-6 c se coloca a 12 cm de una segunda carga puntual de - 1,5 x 10-6 c. calcular la magnitud fuerza eléctrica entre las cargas

In a hydrogen molecule there are a total of four charges, 2 protons in the two nuclei, and 2 electrons. how many unique charge-pairs are there (without double counting)?