Game Development Reference
In-Depth Information
three possibilities will occur depends on the mass, composition, and velocity of the projectile
and target. An arrow shot at a tank will bounce off the tank's armor. The same arrow shot at a
wooden shield might penetrate part or all the way through the shield.
In this section, we'll start by talking about the general physical processes that can occur
during a ballistic impact. We'll discuss energy and momentum issues. Then we'll see some
details about some specific types of armor commonly found in game simulations. Finally, we
talk a little bit about what happens when a bullet or arrow passes through the body of a person
or animal.
What Happens During a Ballistic Impact
A lot of things can happen when a projectile strikes a target at high velocity. For example, let's
consider a bullet that strikes a thick, immovable metal plate. When the bullet strikes the metal
plate, the collision generates a force that acts on the bullet and on the area of the plate that the
bullet impacts. The nose of the bullet will be compressed, and the bullet may experience a
change in its shape due to inelastic (that is, permanent) deformation. Depending on the mate-
rial properties and velocity, the shock caused by the impact may cause the bullet to fragment
into two or more pieces.
The part of the plate that is struck by the bullet will be compressed towards the center of the
plate. The metal in the impact zone will stretch. If the energy of the bullet is large enough,
the plate will fracture, allowing the bullet to penetrate the metal plate. The bullet may even
pass completely through the metal plate and continue its trajectory on the other side. Even if
the bullet does not penetrate the armor, it can still damage the armor, weakening the resistance
of the armor to future ballistic impacts.
Energy Considerations
Ballistic impacts can be analyzed in terms of the overall energy of the projectile and target. You
learned in Chapter 3 that the overall energy of a system must be conserved. Consider a ballistic
impact of a projectile striking and embedding itself into an immovable target. Just before the
projectile impacts the target, it has a kinetic energy equal to one half the mass of the projectile,
m
p
, multiplied by its velocity,
v
p
, squared.
1
2
2
Emv
=
(12.1)
pp
If the projectile penetrates partway into the target and comes to a stop, its kinetic energy is
now zero. We know that energy must be conserved, so where did the preimpact kinetic energy
go? Part of the energy would go into the inelastic deformation of the bullet and target. Friction
between the projectile and target as the projectile was moving through the target would generate
heat. A portion of the kinetic energy would also be expended as work performed by the bullet
when it was penetrating the target. The amount of work performed is equal to the average force
the bullet exerts on the target multiplied by the distance of penetration.
WF x
=Δ
(12.2)
ave
Whether or not a projectile passes through a target depends on whether the projectile has
enough initial kinetic energy to push its way through the target. The amount of penetration, if
it occurs at all, is a function of the material properties of the projectile, the shape of the projectile,
