What is the scientific concept of work?

Answer 1

Work is the displacement of mass.

If you raise a bar bell, you have done work. If you lower it again, total work accomplished becomes zero. So the phrase "work out at the gym" is scientifically a misnomer.

A farmer bringing in bales of hay from a field accomplishes a lot of work. A computer programmer, changing the magnetic orientation of invisibly tiny specs on the metal oxide surface of a rapidly spinning platter accomplishes no work.