First Principles - Sustainable Design: The Science of Sustainability and Green Engineering - page 42

When the forces acting on a body balance one another, the body is at rest.
Let us briefly consider static equilibrium of particles and rigid bodies and discuss
other statics concepts, including moments of inertia and friction, which are
fundamental to green design.
Environmental Determinate Statics
For a rigid body to be stationary, it must be in static equilibrium , which means
that no unbalanced forces are acting on it. Pardon the double negative, but this is
a rare occasion when stating something positively loses some of its meaning. “a
rigid body having balanced forces acting on it” is not the same as “a rigid body
having no unbalanced forces acting on it.”
One of the key concepts in statics that is important to environmental science
and engineering is force. A push or pull by one body on another body is known
as a force . A force is any action that has a tendency to alter a body's state of rest
or uniform motion along a straight line (we discuss Newton's laws regarding
these concepts when we address dynamics and kinetics). Forces come in two
major types, external forces and internal forces. External forces on a rigid body
result from other bodies. An external force may result from physical contact with
another body, known as pushing , or from the body being in close proximity, but
not touching, the other body, such as gravitational and electrical forces. When
the forces are unbalanced, the body will be put into motion. Internal forces are
those that keep the rigid body in one piece. As such, these are compressive and
tensile forces within the body that can be found by multiplying the stress and
area of a part of the body. Internal forces never cause motion but can lead to
deformation. Since force has both magnitude and direction, it is a vector quantity,
so let us discuss vectors briefly as they apply to determinate statics.
The Home Depot Smart Home at Duke University: Rain
Screen and Building Wrap
From ancient times, the concept of shelter has been an expression of fluid
dynamics. Water and air are essential for life, but we need barriers against these
fluids. In fact, our homes have “skins” that selectively allow in air while keeping
water in its liquid state at bay. The Home Depot Smart Home at Duke Univer-
sity has an exterior sheathing called a rain screen (see Fig. SH2.1). The primary
function of the home's exterior sheathing is to prevent water from penetrating
the exterior walls. A rain screen is based on two lines of defense from moisture
penetration. The first is intended to minimize (although not totally eliminate)
the passage of rainwater into the wall. The second is designed to intercept all