Collision

In physics, a collision is an event where two or more bodies exert forces on each other. This interaction happens over a relatively short period of time.

While people often use the word to describe violent crashes, the scientific definition does not depend on the magnitude of the force. Any time objects push on each other simultaneously, a collision occurs. This interaction causes a change in the velocities of the objects involved. The difference in velocity between the objects just before they impact is called the closing speed. Throughout every collision, the total momentum of the objects is always conserved.

Scientists categorize collisions based on what happens to the kinetic energy of the system. Kinetic energy is the energy an object possesses due to its motion. The primary distinction between types of collisions is whether this energy is conserved or lost. If the total kinetic energy remains the same before and after the impact, the event is an elastic collision. In these cases, no energy is released as heat or sound. If kinetic energy is lost or dissipated, the event is an inelastic collision. This lost energy might be absorbed by the objects or turned into other forms of energy.

A perfectly elastic collision is an ideal version of this event. In a perfectly elastic collision, all kinetic energy is conserved. However, this type of collision cannot occur in reality. This is due to the second law of thermodynamics. In contrast, an inelastic collision involves the loss of kinetic energy. For example, when a baseball bat hits a baseball, the kinetic energy of the bat is transferred to the ball. This greatly increases the ball's velocity. The sound produced by the impact represents some of that lost energy.

There is also a specific case called a perfectly inelastic collision, sometimes called a perfectly plastic collision. This occurs when the two bodies coalesce, or stick together, after the impact. A common example is a car crash. Modern cars are designed to crumple inward during a crash rather than bouncing off other vehicles. This design allows the car frame to absorb the energy of the collision. This process is intended to increase safety for the occupants and bystanders. We can measure how elastic a collision is by using the coefficient of restitution. This value typically ranges from zero to one. A perfectly elastic collision has a coefficient of one. A perfectly inelastic collision has a coefficient of zero.

The physics of collisions can be seen in many different scales. In the game of billiards, collisions are nearly elastic. Because billiard balls have low rolling friction, they are often used to demonstrate Newton's laws of motion. If a moving ball hits a stationary ball of equal mass in a zero-friction environment, the balls will move away at a 90-degree angle. This perpendicular movement is a specific result of the conservation of momentum and energy. This mathematical fact is something that professional players must consider during a game.

Collisions also occur on a much smaller scale in science. Collisions between molecules in ideal gases approach perfectly elastic collisions. Similarly, sub-atomic particles can undergo scattering interactions. These particles are deflected by the electromagnetic force. On a much larger scale, some gravitational interactions are almost perfectly elastic. For instance, a satellite can perform a slingshot maneuver around a planet. These large-scale interactions follow the same fundamental principles of energy and momentum found in smaller systems.

Some impacts occur at extreme speeds known as hypervelocity. Hypervelocity is defined as a velocity exceeding approximately 3,000 meters per second. At these speeds, the strength of materials becomes very small compared to inertial stresses. This causes metals and fluids to behave in a similar way during impact. If the velocity is extreme enough, the impactor and the target may undergo vaporization. This can turn solid matter into gas. NASA demonstrated this by crashing the Deep Impact probe into the comet Tempel 1. Meteorite impacts that create craters on planets are also examples of hypervelocity events.

Finally, collisions are a key part of how living things move. When an animal's foot or paw hits the ground, it creates ground reaction forces. These are considered inelastic collisions because kinetic energy is not conserved. Researchers study these forces to understand the gait of both disabled and non-disabled individuals. This work often involves using a force platform, also known as a force plate. By measuring these collisions, scientists can better design prosthetics to help people walk more naturally.