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Newton's Third Law of Motion codifies something we see all the time, namely that when object A impresses a force on object B, object B reacts by impressing a force back on object A that is of equal magnitude, but in the opposite direction. Newton expressed this law in a straightforward fashion, saying that "for every action, there is an equal and opposite reaction".
There are three ways that an object can accelerate: 1) its speed changes (but not its direction), 2) its direction changes (but not its speed), or 3) both its speed and direction change.
Since an object requires a net non-zero force for its velocity to change, we say that the object has inertia, a resistance to a change of its velocity. The more mass an object has, the more inertia it has.
Newton's Second Law of Motion is the converse of the First Law: if an object experiences a net, non-zero external force, then its velocity must change: it must accelerate.
Newton's First Law of Motion states that an object's velocity (speed and direction) remains unchanged, unless a net, non-zero external force acts upon it. Think of a hockey puck traveling the entire length of an ice rink without hitting something: it naturally travels in a straight line, with unchanging speed.
Isaac Newton is credited with discovering three Laws of Motion that can be used to explain all the motions of celestial (or Earth-bound) objects.
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