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Steel is an Allomantic metal and an alloy of Iron. When burnt, Steel allows the user to telekinetically push against a metal object, presumably

propelling that object in the opposite direction. Normal laws of physics (Force = Mass * Acceleration, Momentum = Mass * Velocity, and the conservation of momentum -- for every action there is an equal and opposite reaction) still apply here, so if you push against a massive object, you will need to apply more force to make it move. If you push against an object similar in mass to you, you may move as well, and if an object is much more massive than you, you will be pushed away from it. As a result, many allomancers use steel to allow themselves to fly through the air. A common use is to drop a coin on the ground, and then push on it. Because the coin is against the ground, steel-pushing on it is effectively the same as pushing on the ground, which is exponentially more massive than the Allomancer, so the resulting acceleration primarily affects the allomancer, allowing them to "fly" through the air. Allomancers also push on coins to use them as projectile weapons. Other uses for steel-pushes include pushing upon a person that possesses a metal object, or disarming a person with a metal weapon. Small-bodied allomancers are at a distinct tactical disadvantage in combat as their steel-pushes require them to brace themselves either physically or allomantically before pushing against larger people. Mistborn can avoid crushing themselves by burning pewter to strengthen their bodies. Any misting only capable of burning steel is known as a coinshot.

When used in Feruchemy, steel allows the user to store physical speed, becoming slow and sluggish. When tapping steel however, their speed, as well as their reaction times, are sped up. When tapping steel, it is possible for a Feruchemist to run incredibly fast, faster even, than an Allomancer on a Pewter-drag, though it is unlikely that a Feruchemist could maintain these speeds for as long due to the stores of speed required.  

When used in Hemalurgy, steel will transfer the ability to burn a single physical metal (iron, steel, tin, pewter) Allomantically.