Game Development Reference
In-Depth Information
Aerodynamic Drag
Aerodynamic drag is the resistance that air or any other type of gas exerts on a body traveling
through it. If you stick your hand out of the window of a moving car or try to ride your bicycle
into the wind, you can feel the drag force on your body. Drag is an important physical effect,
particularly for high-speed projectiles such as bullets, golf balls, or even parachutists. The next
level of complexity we will add to our projectile model will be to account for aerodynamic drag
forces and model how they affect the trajectory of a projectile.
Basic Concepts
Drag is a force that resists motion through a fluid medium. Like frictional force, drag force acts
in the opposite direction to the velocity vector of an object. If a projectile is moving to the right,
the drag force on the body will be directed towards the left. For purposes of analyzing projectile
motion, there are two components to the total drag force on a projectile. The first drag compo-
nent is due to pressure. The surface pressure on the front of an object traveling through the
fluid will be greater than the pressure on the back of the object. This pressure difference creates
a net force, or
pressure drag
, on the object.
The second component of drag on a projectile is due to the friction. In Chapter 3, you
learned how friction is a force that resists the motion of two objects sliding against each other.
With a projectile in flight, a frictional force develops between the surface of the projectile and
the air or other fluid that is sliding past it. This frictional force is commonly referred to as
friction
drag
or
skin drag
.
The total drag on a projectile is the sum of the pressure and friction drag components.
FF
=
+
F
(5.11)
D
D pressure
,
D friction
,
It turns out there are other components of drag that become important in modeling the
flight of an airplane, but we'll defer discussion of that topic until Chapter 10.
The magnitude of the drag force on an object is a function of the geometry of the object,
the density of the fluid,
r
, in which it is traveling, and the square of the velocity,
v
. Drag force is
usually expressed as a function of these terms and a quantity known as a
drag coefficient
,
C
D
.
1
---
r
v
2
AC
D
F
D
=
(5.12)
