Unit 1 - Lesson 3: Motion in One Direction: Uniform, Average & Relative Speed

Motion is defined as the change in an object's position over time relative to a fixed reference point. The simplest type of motion is motion in a straight line in one direction (such as the motion of a train along a straight track). To describe and measure motion, physics relies on two primary physical quantities: distance (d) covered and time (t) taken. Speed (v) is defined as the distance covered per unit of time, or the rate of change of distance: [v = d / t]. The standard International System (SI) unit for speed is meter per second (m/s) or kilometer per hour (km/h). To convert from km/h to m/s, multiply by (5 / 18); to convert from m/s to km/h, multiply by (18 / 5).

An object moves at (Regular / Uniform Speed) when it covers equal distances in equal intervals of time, regardless of how small these time intervals are. In real-world vehicular traffic, moving at a constant uniform speed is practically impossible due to varying road conditions, intersections, traffic lights, and congestion. Therefore, cars and vehicles generally travel at (Irregular / Non-Uniform Speed), covering unequal distances in equal intervals of time, or equal distances in unequal intervals of time.

To describe motion at irregular speed with mathematical precision, physics uses (Average Speed, v_avg). Average speed is defined as the total distance covered divided by the total time taken to cover that distance: [v_avg = Total Distance / Total Time]. Average speed represents the uniform speed at which an object would move to cover the same total distance in the same total time. When an object's average speed strictly equals its instantaneous speed at every point along the journey, its motion is uniform; when they diverge, the motion is non-uniform.

The speed of a moving object as estimated or measured by an observer is called (Relative Speed). Relative speed depends entirely on the state and direction of the observer: 1. (Static Observer): Relative speed equals the object's actual speed: [v_rel = v_actual]. 2. (Observer moving in the Opposite Direction): Relative speed equals the sum of the two speeds: [v_rel = v_actual + v_obs]. The object appears to move much faster. 3. (Observer moving in the Same Direction with Different Speed): Relative speed equals the difference between the two speeds: [v_rel = |v_actual - v_obs|]. 4. (Observer moving in Same Direction with Identical Speed): Relative speed equals zero: [v_rel = 0]; the moving car appears stationary to the observer.