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 the shield?
Captain America throws his shield with a defined force. The only thing that will slow down the shield is any form of drag force, or collisions with other bodies. It is essentially just a giant frisbee of death.