Speed

Introduction

When an object moves from one place to another, it is said to be in motion. There are a number of ways in which motion can be described. One of these ways is speed.

Definition of speed

Speed is the distance travelled compared with the time it takes to travel that distance. Distance is the space through which an object travels. In the diagram, for example, the distance between point A and point B is 100 metres (m). Speed, therefore, measures how quickly an object moves from one point to another.

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There are two ways speed can be described: instantaneous speed and average speed. Instantaneous speed is the speed at which an object is travelling at a certain point in time. This is the type of speed that is measured by a speedometer or a radar gun. Speed rarely remains constant over longer periods of time, however. Average speed is the overall speed of an object between two points. Your instantaneous speed on some parts of the drive to school might be around 50 kilometres per hour (km/h), for example, but your average speed would be lower because you would have to stop at traffic lights, slow down to turn, and so forth.

Average speed can be expressed in the following equation:

Let's say a person is able to walk 100m in 50 seconds. 100 divided by 50 is 2, so the person's average speed would be 2 metres per second (m/s). Speed is always measured in a unit of distance divided by a unit of time. Some examples are metres per second (m/s), kilometres per hour (km/h) and miles per hour (mph). In physics, m/s is the most commonly used unit of speed.

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Average speed and instantaneous speed are often different, but if smaller units of measurement are used in the equation, a speed closer to the instantaneous speed can be found. From our example, if we were able to measure how quickly the person walked 10m or 1m, we would have a number closer to the person's instantaneous speed.

Distance-time graphs

In order to better understand speed, it can be helpful to look at a distance-time graph. A distance-time graph plots distance on the vertical (y) axis and time on the horizontal (x) axis.

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In this diagram, line A represents an object travelling at a slow, constant speed. Line B represents an object travelling at a quicker speed, while line C represents an object travelling at a constant speed, then stopping.

The speed of an object can be determined from a distance-time graph by finding the slope of the graph. This is done by picking two points on the graph and drawing a straight line down from the higher point and a straight line to the right from the lower point. Determine the lengths of these lines. The slope, and therefore the speed of the object, is the length of the vertical line (called the rise) divided by the length of the horizontal line (called the run).