What do you mean by 'back emf is the answer'.
Do you know how to calculate back emf?
what is your background knowledge, your profile gives no clues, what text books on this topiuc have you referred to?....were they of no help?
I'm just a guy off the street with some layman knowledge. I never said my answer is the final answer, and I clearly indicated that this is simply "my understanding". I then went on to offer an explanation that gives insight into why I believe this is the answer. I expect OP won't take my word as gospel, but will wither use it to direct his own research or will seek confirmation from more qualified sources.