From a Boyhood Catapult to the
Forensic Language of Impact: A Reflection on Energy, Pressure, and the Memory
of Matter
There is something profoundly
poetic in the way my early fascination with a simple catapult has led me into
the deeper truths of physics and forensic science. What seemed like a child’s
play launching stones against tin cans and observing the dents—was in reality
an intuitive exploration of nature’s laws. Those small indentations were not
merely marks on metal; they were physical records of energy transfer, force
concentration, and material response.
At the heart of all motion and
impact lies a simple yet powerful relationship:
This equation tells us that
kinetic energy depends not only on mass, but more significantly on velocity.
The squaring of velocity means that even modest increases in speed result in
dramatic increases in energy. This is why a small, fast-moving bullet can rival
or exceed the effects of much heavier but slower-moving objects when it comes
to penetration and localized damage.
Let us return, for a moment, to
that mischievous boy (me) playing with the catapult in school
A small stone of mass 
,
travelling at about 
, carries
an energy of:
This is a modest amount of
energy, yet when the stone strikes a tin surface, it stops over a very short
distance. Using the relationship:
and assuming a stopping distance
of 
, the force
becomes:
When this force is applied over
a small contact area, say 
, the
resulting pressure is:
It is this concentration of
force into a small area that produces the visible dent. Even with relatively
low energy, the pressure is sufficient to exceed the yield strength of softer
metals. What I observed was, in essence,
the threshold at which matter begins to permanently deform.
Now, as we move into the realm
of firearms, the same principles apply but on a vastly different scale.
A typical 9mm bullet, carrying
about 
of energy,
comes to rest over a much shorter stopping distance, approximately 
. Applying
the same reasoning:
If this force is distributed
over a contact area of roughly 
, the
pressure becomes:
Here, we enter a regime where
the pressure exceeds the strength of most structural metals. The bullet
deforms, flattens, and transfers its energy efficiently into the target. The
deformation itself is part of the energy dissipation process, and the resulting
marks, flattening, radial cracks, and
indentation patterns—become valuable forensic clues.
When we consider a high-velocity
rifle bullet, the situation becomes even more dramatic. With an energy of
approximately 
, and an
even shorter stopping distance of 
, we find:
With a smaller contact area of
about 
, the
pressure rises to:
At such extreme pressures,
materials behave in ways that defy everyday intuition. Steel may momentarily
flow, microscopic cracks propagate violently, and shock waves travel through
the material. The impact resembles, in some respects, a miniature meteorite
collision. The resulting damage, penetration, fragmentation, and spalling, leaves
behind a complex signature that can be studied and interpreted.
And yet, when we shift our
attention to something as massive as a car, the story changes.
A car of mass 
,
travelling at 
,
possesses:
This is hundreds of times
greater than the energy of a bullet. However, because this energy is
distributed over a large area and dissipated over a longer stopping distance,
the resulting pressure is far lower. Instead of piercing, the car deforms and
crushes. This illustrates a central truth: it is not energy alone, but the
concentration of energy in space and time that determines the outcome of an
impact.
To understand how materials
ultimately fail, we turn to fracture mechanics. The critical relationship is
expressed as:
This equation describes how
stress (
) interacts
with microscopic cracks of length 
within a
material. When the stress intensity factor 
exceeds the
material’s fracture toughness, cracks propagate rapidly, leading to failure.
High-velocity impacts dramatically increase ,
activating even the smallest imperfections within the material.
High-velocity impacts
dramatically increase σ, activating even the smallest imperfections within the
material. What appears to be a solid, uniform surface is, at the microscopic
level, a landscape of potential fracture points waiting to be triggered.
It is here that physics meets
forensic science - an area I am familiar at Cambridge
Every impact leaves behind a
record—a silent testimony encoded in deformation, cracks, and residue. By
examining penetration depth, investigators may estimate energy using:
By analysing momentum:
and changes in momentum:
they can infer whether a bullet
ricocheted, penetrated, or fragmented. The angle of impact, the type of weapon,
and even the distance from which the shot was fired can often be reconstructed
from these physical clues.
Surface features tell their own
story:
- Smooth dents suggest lower pressures and
ductile deformation
- Sharp fractures indicate high strain
rates and brittle failure
- Spalling reveals intense internal stress
waves
Thus, the material itself
becomes a witness.
And so, dear readers we come
full circle.
The young boy- that’s me
observing dents in a tin with a catapult was already engaging with the
same principles that modern forensic scientists use to reconstruct events from
the smallest traces. What began as curiosity has matured into understanding,
and what seemed like simple play has revealed itself as an encounter with the
fundamental laws governing matter and motion.
There is a quiet elegance in
this continuity that the same physics governs a stone from a catapult, a bullet
from a rifle, and the conclusions drawn in a forensic laboratory. The language
is consistent; only the scale changes.
And perhaps the most beautiful
insight of all is this:
Matter remembers.
Every impact, no matter how brief, leaves behind a story waiting for a mind as
curious as anyone who read it - just to share my memories of yesteryears
