Sir Isaac Newton first presented his three laws of motion
in the "Principia Mathematica Philosophiae Naturalis" in 1686. His first law
states that every object will remain at rest or in uniform motion in a straight
line unless compelled to change its state by the action of an external force.
This is normally taken as the definition of inertia. The key point here
is that if there is no net force resulting from unbalanced
forces acting on an object (if all the external forces cancel each other out),
then the object will maintain a constant velocity. If that velocity is zero,
then the object remains at rest. And if an additional external force is applied,
the velocity will change because of the force. The amount of the change in velocity
is determined by Newton's second law of motion.
There are many excellent examples of Newton's first law involving aerodynamics.
The motion of a
is described by the first law.
When the forces of weight
are all in
there is no net
external force on the glider and it continues to fly at a constant velocity.
Since a glider has no engine, this motion can only occur when the aircraft is
and some component of the weight vector counter-acts the drag.
- Re-Living the Wright Way
- Beginner's Guide to Aeronautics
- NASA Home Page