Newton's Laws of Motion

Newton’s first law: The Law of Inertia

            An object in motion tends to stay in motion, and an object at rest tends to stay at rest, unless the object is acted upon by an outside force.

Newton’s second law: The Law of Acceleration

            The acceleration with which an object moves is directly proportional to the magnitude of the force applied to the object and inversely proportional to the mass of the object.

          In terms of equations:    a=F/m    or   F=ma    where a = acceleration, F = force and m = mass

Newton’s third law: The Law of Interaction (The Action-Reaction Law)

            For every action there is an equal and opposite reaction.

Further analysis of the Action-Reaction Law

              The third law stated is correct, but it sometimes can be a little confusing.  To actually know what the law means we must understand the meaning of the words “action…reaction”, “opposite” and “equal”.

        “Action…reaction” means that forces always occur in pairs.  Single, isolated forces never happen! The two forces involved are  called the ‘action force’ and the ‘reaction force’.  These given names are confusing for two reasons:

1)     Either force in an interaction can be the ‘action’ force or the ‘reaction’ force.

2)     Unfortunately we associate ‘action’ and ‘reaction’ with ‘first an action, then a reaction’.

This is NOT what occurs in the third law.  The action force and the reaction force exist at the SAME time.

               “Equal” means two things:

1)     Both forces are exactly the same size.  They are equal in magnitude.

2)     Both forces exist exactly at the same time.  They are equal in time.

             “Opposite” means that the two forces always act in opposite direction, i.e. exactly 180 degrees apart.