What is Average Speed?
Speed is a measure of how fast an object is moving. It is a scalar quantity, which means it only has a magnitude (a numerical value) and no direction. For example, "50 km/h" is a speed.
Average speed ($s$) is defined as the total distance an object travels divided by the total time it takes to travel that distance. It doesn't account for any variations in speed, such as speeding up, slowing down, or stopping, during that time interval. It's simply the overall average over the entire journey.
The Average Speed Formula
The formula for calculating average speed is simple and fundamental:
- $s$: Average speed, measured in meters per second (m/s).
- $d$: Total distance, measured in meters (m).
- $t$: Total time, measured in seconds (s).
Speed vs. Velocity
This is one of the most common points of confusion in physics. The key difference is direction:
- Speed (Scalar): Refers to "how fast" an object is moving. It depends on the total distance traveled. If you run around a 400m track and end up where you started, you traveled a distance of 400m, and you had an average speed.
- Velocity (Vector): Refers to the rate at which an object changes its position (or displacement). In the track example, since you ended where you started, your final displacement is zero. Therefore, your average *velocity* is zero, even though your average *speed* is not.
An object's speed can be constant while its velocity changes. For example, a car driving in a circle at a steady 30 km/h has a constant speed. However, its velocity is constantly changing because its direction is always turning. Any change in velocity is an acceleration.