Impact velocity is essentially the effective velocity at which an object is traveling at a moment in which it impacts another object. This can be expressed somewhat simplistically for situations in which outside forces are being ignored or are not present as the absolute value of the sum of the velocities of both objects. When other forces on the two objects are considered, such as the force of gravity, friction, wind resistance and similar outside forces, then the value of the velocity at the moment of impact is significantly more complicated. Impact velocity can be determined for any two objects that come into contact with each other.
In order to easily understand impact velocity, it is important to first understand what velocity refers to in general. Velocity is a measurement of the distance an object can travel over a given period of time and is typically measured in meters per second (m/s). While velocity is similar to speed, the two terms cannot necessarily be utilized synonymously since velocity can also indicate the direction in which an object is traveling relative to another point of reference. This means that an object with positive velocity and an object with negative velocity within the same system are traveling in opposite directions; speed is not indicative of direction and typically cannot be a negative value.
The impact velocity of an object is essentially the velocity at which an object is traveling when it comes into contact with another object. For example, if a rock is traveling in a vacuum devoid of other forces upon it, and it impacts another rock that is at rest, then the impact velocity is the velocity of the rock when it strikes the rock at rest. If the rock in the previous example was traveling at 100 m/s and struck another rock that was traveling at 40 m/s toward it, then the impact velocity would be 140 m/s since the absolute value of both velocities are added together to determine the effective velocity at the moment of impact.
In more complicated systems, determining the impact velocity can become more difficult. This is because other forces often play upon an object, and over time these forces can affect the velocity at which it is traveling. A bullet fired downward at an angle from a rooftop toward an apple sitting on the ground, for example, would have an initial velocity when it is fired and be affected by the force of gravity, which would increase its velocity by pulling it downward. Wind resistance against the surface of the bullet would also alter the impact velocity once the bullet struck the apple, and all of these forces would need to be considered to create an accurate model for this type of system.