2.2: Instantaneous and Average Values for Physical Quantities
An instantaneous value is a value measured at a given instant, or time. For example, we can measure the velocity of an object right at high noon as 5.5 km/s east. This is an instantaneous value because we measured it at a given instant in time. A car's speedometer is an example of an instantaneous measurement. Furthermore, velocity does not necessarily stay constant over time, so instantaneous measurements can vary depending on when you take the measurement.
An average value is calculated over a period of time. For example, to calculate average speed, divide the distance traveled by time traveled. For example, if you drive 30 miles in two hours, your average speed is 15 miles/hour. However, as we know from driving, we rarely drive exactly the same speed for two hours. So, the instantaneous value of your speed could vary at any given time, but the average value is still 15 miles/hour.
Read this text which explains what we mean when we talk about instantaneous versus average time, velocity, and speed. Note that we will explore the calculations you see in this text in detail in Section 2.4 below. For now, make sure you understand what these concepts mean before we begin discussing distance and displacement next.