# Law of inertia, Frames of reference

Ok, we all know that an Inertial frame of reference is defined like this:

"An inertial frame of reference is one frame where Newton's First Law holds, therefore, a body has a constant velocity or velocity equal zero. And, the sum of all forces equals zero, there is no aceleration etc..."

But, I always read that in an inertial frame of reference holds $F=ma$, the second law.... It's confuses me... why we can assume $F=ma$ in an inertial frame of reference?

• A couple of things. (A) Keep in mind that the $F$ in Newton's 2nd law is the net force. (B) Consider what Newton's 2nd law says in the case of $F = 0$. – dmckee Oct 18 '15 at 16:37