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 remains 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 acting on an object
(if all the external forces cancel each other out) then the object
maintains a constant velocity.
If that velocity is zero, then the object remains at rest.
If the velocity is not zero, then the object maintains that velocity and travels
in a straight line.
If a net external force is applied, the velocity changes because of the force.
The liftoff of a rocket from the
launch pad is a good example of this principle. Just prior to
engine ignition, the velocity of the rocket is zero and the rocket is at rest.
If the rocket is sitting on its fins, the
weight
of the rocket is balanced by the reaction of the earth
to the weight as described by Newton's
third law of motion.
There is no net force on the object, and the rocket would
remain at rest indefinitely. When the engine is ignited, the
thrust
of the
engine creates an additional force opposed to the weight.
As long as the thrust is less than the weight, the combination of the
thrust and the reaction force through the fins balance the weight and
there is no net external force. The rocket stays on the pad.
When the thrust is equal to the weight, there is no longer any reaction
force through the fins, but the net force on the rocket is still zero.
When the thrust is greater than the weight, there is a net external force
equal to the thrust minus the weight, and the rocket begins to rise.
The velocity of the rocket increases from zero to some positive value
under the acceleration produced by the net external force.
As the rocket velocity increases, it encounters air
resistance, or aerodynamic drag,
which opposes the motion.
Drag increases as the square of the
velocity. The thrust of the rocket must be
greater than the weight plus the drag for the rocket to continue
accelerating. If the thrust becomes equal to the weight plus the drag,
the rocket continues to climb at a fixed velocity, but it does not
accelerate. This flight condition is often encountered by
model rockets
because of the low thrust and high drag of their design.
Full scale rockets usually have sufficient excess thrust to
continue accelerating. Drag eventually begins to decrease because
drag depends on the air
density and density
decreases with increasing
altitude.
Guided Tours

Newton's Laws of Motion:
Activities:
Fundamental Terminology: Grade 1012