Through the wonders of Conservation of Momentum
You may be wondering, "what is conservation of momentum"? The Law of Conservation of Momentum states that "the momentum of a system is constant if there are no external forces acting on the system." (Motta, 2007)
To understand this statement, we must define momentum. Dictionary.com defines momentum as "Force or speed of movement."
To understand this statement, we must define momentum. Dictionary.com defines momentum as "Force or speed of movement."
Okay but that sounds really science-y and confusing
Basically it comes down to Newton's first law, "An object in motion tends to stay in motion and an object at rest tends to stay at rest unless acted upon by an external force. If you're in space, and you try to swim with nothing to push against, you won't go anywhere.
Okay, Okay, I get it. But how does that apply to Thor?
If Mjolnir was permanently attached to Thor's arm, he wouldn't be able to fly, he'd just be spinning his arm around. But since Thor can let go of his hammer, he can push himself(attached by friction to the Earth) away from the hammer. If you were in space and you needed to get home, you would throw things in an opposite direction of where you're trying to go, because the reaction force would push you back.
So...Thor's hammer makes him fly by pushing him into the Earth??
The consensus is that Thor lets go of his hammer, and then grabs it a split second afterward. This gives Mjolnir time to emit gravitons and increase its mass, thus increasing its momentum. When he grabs on to it, it is like a perfectly inelastic collision between him and the hammer, and the hammer just pulls him along for the ride.