* Newton's First Law of Motion (Inertia): The hammer, initially at rest, requires a force (the swing of the user) to set it in motion. Once the hammer is in motion, it continues to move in a straight line at a constant speed until an external force acts upon it (the impact with the nail).
* Newton's Second Law of Motion (Force and Acceleration): The force applied to the hammer (the swing) causes it to accelerate towards the nail. The heavier the hammer, or the faster it's swung, the greater the force and acceleration. This force, applied to the nail, causes it to accelerate into the board.
While Newton's Third Law of Motion (Action-Reaction) is also at play (the hammer exerts a force on the nail, and the nail exerts an equal and opposite force on the hammer), it's less directly demonstrated in the simple act of hammering.
