A very basic concept when dealing with
forces
is the idea of equilibrium or balance.
In general, an object can be acted on by several forces
at the same time.
A force is a
vector quantity
which means
that it has both a magnitude and a direction associated with
it. Two forces with the same magnitude but different directions
are not equal forces.
The vector sum
of all of the forces acting on a body is
a single force called the net force .
If the net force is equal to zero,
the object is said to be in equilibrium.
Because there is no net force acting on an object in equilibrium,
then from Newton's
first law
of motion, the object continues to
move
at a constant speed.
On another page, we show the simplest example of equilibrium with
two forces acting on an object.
On this page, we will consider the case of a
glider, which has three forces acting on it in flight.
And on another page we consider the case of
powered aircaft in
cruise, where four forces act on the airplane.
In Example 1, we show a computer drawing of a
glider as it is descending. There are three forces acting on the glider;
lift (L),
drag (D),
and
weight (W).
The weight is always directed towards the center of the earth, the lift
is directed perpendicular to the flight path, and the drag is along the flight
path. The flight path is inclined to the horizontal at an
angle a.
When the aircraft is in equilibrium, the vector sum of these three forces
is equal to zero. Because it is a vector sum, there are two
component
equations, one vertical and one horizontal, which are
shown below the graphic.
W  L * cos(a)  D * sin(a) = V = 0
D * cos(a)  L * sin(a) = H = 0
where sin and cos are the
trigonometric sine and cosine functions,
V is the net vertical force, and H is the net horizontal force.
Because we are looking at an equilibrium condition, H and V are equal to zero.
The aircraft has a constant forward and downward velocity along
the flight path. Notice that the lift, drag, and weight all continue to
act on the aircraft. In equilibrium, the action of some forces are exactly
balanced, or cancelled out, by other forces.
In Example 2, a
spoiler
is deployed on the top of the glider's wing, decreasing the lift and increasing the drag.
The weight remains the same.
The glider is no longer in
equilibrium. The equations for the forces remains the same, but there are now
net horizontal and vertical forces; V and H are not equal to zero.
According to Newton's
second law
of motion, the aircraft would begin to accelerate downward and to the right. In Example 2, the
forces are not balanced and the airplane is not in equilibrium.
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