Hello, this is my first post. Some years back i read a book about einstein works. Among many things in the book, i read about his 'modified' newton's equation of motion (F=ma). Sadly, i had lost the book and can't recall the equation. All i can remember is it says that the einstein's equation should be used when a body is moving at close to or higher than speed of light. The F=ma is still correct for motions slower than that. I hope someone can tell me what is the equation. Thanks. p/s: English is not my 1st language, so i hope everbody can understand what i'm trying to say here.