In the scene where Cooper ejects himself into space, towards Gargantua, he quotes Newton's Third Law of Motion:
"You have to leave something behind to go forward"
We all know that that Law states:
"for every action there is an equal and opposite reaction"
Well the question is that he's basically dumping fuel to get a slingshot to go into Gargantua, but instead I believe that rather than referring to technical details, Cooper is actually hitting our emotional nerves - in his last chance to save earth. I believe that that's why he said the quote as he knew he was going for an everlasting sacrifice for humanity.
What does he actually mean in the scene?