What Really Happens During a Collision? A Physics Breakdown

Collisions are everyday events that range from a football hitting a goalpost to vehicles crashing on the road. While they may seem chaotic and unpredictable, the science behind collisions is grounded in predictable laws of motion and energy. Understanding what really happens during a collision is a fundamental topic in physics, especially in mechanics.

Read on as we offer a detailed breakdown of the processes and principles at play when objects collide, helping you grasp the fascinating forces involved.

Types of collisions: elastic and inelastic

All collisions fall into two main categories: elastic and inelastic. In an elastic collision, both momentum and kinetic energy are conserved. This means that after the collision, the objects bounce off each other without any loss of total kinetic energy. Perfectly elastic collisions are rare in the real world but are useful in theoretical problems and models. Gas particles in an ideal gas, for example, are often assumed to undergo elastic collisions.

In contrast, inelastic collisions occur when kinetic energy is not conserved. Instead, some of it is transformed into other forms of energy, such as heat, sound, or deformation. A special case of this is a perfectly inelastic collision, where the colliding objects stick together after impact. Car crashes, clay blobs colliding, and sports tackles are real-world examples of inelastic collisions.

The conservation of momentum explained

One of the most critical principles in understanding collisions is the conservation of momentum. This law states that the total momentum of a closed system remains constant, provided no external forces act on it. In a two-object system, the total momentum before and after the collision must be equal.

For example, if a stationary object is struck by a moving object, the system’s total momentum must still be the same after the impact. This means the moving object will slow down, speed up, or transfer some of its momentum, depending on the masses involved and the type of collision.

Students often explore this concept during practical experiments, such as using air tracks or trolleys to demonstrate how momentum is shared and transferred. This is a core topic in Physics tuition, as it builds the foundation for more advanced study in mechanics and real-world applications.

Force, impulse and collision duration

Collisions don't happen instantaneously. They occur over a very short time period, during which forces act between the objects. The concept of impulse is essential here. Impulse is the product of force and the time over which it acts, and it equals the change in momentum of an object.

If the time of contact during a collision increases, the force experienced decreases. This principle is why crumple zones in cars and cushioned sports mats are so effective. They extend the time over which the collision occurs, reducing the peak force and thereby minimising injury or damage.

This connection between impulse and momentum provides insight into how everyday safety features work. It also shows how physics plays a crucial role in engineering and design to protect people from harmful impacts. In fact, physics helps us design safer vehicles by informing decisions on materials, structures, and energy absorption mechanisms.

Energy transformation during impact

Kinetic energy, although not always conserved in inelastic collisions, plays a key role in understanding how energy transforms. In most real-world collisions, part of the initial kinetic energy is converted into other energy forms. This energy might produce sound, heat, or deformation. That’s why when two cars collide, you often hear a loud noise, feel heat from friction, and see structural damage.

In elastic collisions, on the other hand, objects rebound with no loss in kinetic energy, meaning all the energy remains within the system in the form of motion. Billiard balls provide a good example of nearly elastic collisions, where energy is conserved to a large extent and the balls continue to move post-impact.

Understanding how energy changes form during collisions allows students to think critically about real-life applications and engineering solutions.

The role of mass and velocity

The outcome of any collision is heavily influenced by the mass and velocity of the colliding objects. According to Newton’s second law, force is proportional to mass and acceleration. Therefore, a heavier object moving at a higher velocity will generally exert more force in a collision.

Momentum, which is the product of mass and velocity, helps determine how two objects will move post-collision. If a fast-moving object hits a stationary lighter one, the lighter object will typically move away with a greater speed. Conversely, if the lighter object collides with a heavier one, it might bounce back or come to a stop, depending on the energy and direction of the impact.

These dynamics are central to many physics problems and are often visualised using vector diagrams and equations. Having a strong understanding of these principles makes solving mechanics questions more intuitive and logical.

Real-life applications of collision physics

Beyond academic interest, understanding collision mechanics has direct relevance to real-world industries. Engineers designing cars, sports equipment, helmets, and even packaging materials rely on collision physics to enhance safety and efficiency. In sports, knowledge of how momentum and impact force work can improve athletic performance and reduce injuries.

Space exploration, robotics, and construction also benefit from these principles. Designing machines that interact with other objects, land safely, or absorb shocks requires accurate predictions about how collisions will occur and how to minimise their damaging effects.

Learning collisions through Physics Tuition

Collisions may appear simple on the surface, but they involve a web of interrelated concepts, including momentum, energy conservation, impulse, and force. Mastering these principles requires a strong grasp of both theory and application.