By definition, social science is the study of human society and human relationships. This discipline is generally thought of as consisting of seven disciplines: economics, geography, law, anthropology, archaeology, psychology, history, linguistics, politics, and sociology. Social justice is a philosophical and political idea that everyone has an equal right to wealth, advantages, and opportunities.
When it is a philosophical theory, there is no issue and no need for social science. However, when a group, whether that is a government or a grassroots activist organization, attempts to act on a social justice concept, they have to define it, and each group has defined it a bit differently. Economics becomes involved in many ways; some groups want to redistribute wealth completely, others want to make new social justice programs, but there will need to be an economic plan to pay for them. Law and politics are self-explanatory, I think. History, sociology, anthropology, and psychology almost as much since people try to understand why people do what they do. Socialism and communism are political systems that were born out of an idea of social justice, so we study them to understand how a concept that has so many good things in it, went so wrong. Linguistics and geography are not as simple to peg into social justice as their own fields, but they fit within the other disciplines as professionals travel or communicate with others to do their work. It would not even have to be someone of another language to need the help of a linguist. They can help people communicate right here at home.
I hope this has answered your question. Feel free to reach out to me if you have any further questions or a follow-up.