Forensic physics in a hit-and-run case

Hey there, science enthusiasts and true crime buffs. If you have ever watched a crime show where experts analyse bullet trajectories or reconstruct car crashes, you’ve glimpsed the fascinating world of forensic physics. But what exactly it is and how it’s crucial. In this blog post, I will delve into hit-and-run cases and the mystery they pose. No witness, just a mess of evidence on a road. Let’s break it down into simple steps, as if we’re chatting over coffee.

I will keep the science stuff simple.

It starts with a hit-and-run case. Forensic physics and incident. Reconstruction in the 2023 Kanjhawala case. Anjali Singh was dragged under a car for over 12 km, utilising Locard’s exchange principle to link the victim to the vehicle. Forensic experts recreated the scene, identifying that the victim’s leg got stuck in the car axle, with dust and bloodstain pattern from the road confirming the drag path. Examination of the vehicle’s undercarriage (axle/suspension) allowed investigators to determine how the body was trapped and dragged for a long distance. This case was solved by forensic investigators.

Below, I will break it down step by step, showing how it works in a real investigation.

1. Initial Scene Assessment and Skid Marks Analysis

Key physics principle: Friction and deceleration.

How it helps: Skid marks on the road are like a “fingerprint” of the accident. Physicists measure the skid mark’s length, width, and pattern to calculate the vehicle’s speed before braking. For instance, longer skid marks indicate higher speeds or poorer road conditions. To estimate these speeds, investigators use the fundamental formula for deceleration:

$v^2 = u^2 + 2as$

(Where v is final velocity, u is initial velocity, a is acceleration, and s is distance)

2. Vehicle Damage and Impact Reconstruction

Key physics principle: Conservation of momentum and energy transfer.

How It Helps: By examining dents, crumple zones, and debris, experts model the collision. They calculate force vectors to determine the angle of impact, vehicle weights, and speeds. Software like PC-Crash simulates the event, factoring in variables such as tyre pressure and road incline.

3. Trajectory and Ejection Analysis

Key Physics Principle: Projectile motion and Newton’s laws of motion.

How It Helps: If a pedestrian or cyclist is struck, physicists can trace their flight path using launch angle, velocity, and gravity. Blood spatter patterns or body positioning can confirm impact points. To map these distances and understand the force of the impact, investigators use the projectile range equation:

$R = \frac{u^2 \sin(2\theta)}{g}$ $R = \text{projectile range, } u = \text{initial velocity, } \theta = \text{launch angle, } g = \text{acceleration due to gravity}$

4. Material Evidence and Trace Analysis

Keys Physics Principle: Material properties and optics

How It Helps: Paint transfers, glass shards, or tyre tread impressions are analysed to determine a match. Physicists use microscopy and spectroscopy to identify unique signatures, such as the refractive indices of glass or the chemical composition of paint.

Forensic physics relies on assumptions, so results aren’t infallible. Experts must account for these in court, often using probability models. Collaboration with accident reconstruction specialists ensures accuracy.

In summary, forensic physics transforms chaotic hit-and-run scenes into data-driven narratives, often tipping the scales of justice.