How to Calculate Average Speed

How to Calculate Average Speed

Maybe you are planning a road trip and want to know how fast you must travel to reach a destination by a certain deadline. Or perhaps you are curious and want to know the average speed of a train traveling between two points. Or just maybe you need to finish a homework problem. Whatever the reason, it is useful to know how to calculate average speed.

Part1

Learning the Equation

1.Understand the definition of speed. 

Speed can be defined as "the rate at which an object covers a distance." Necessarily, speed is described using a combination of distance and time measurements. To clarify, there are two other related terms that you should understand as well.

·         Velocity (in physics) is very similar to speed, but differs in an important way. Formal descriptions of velocity must include the vector (direction) along which the object moves. Because of this, velocity is calculated through a slightly different formula. Unlike speed, velocity can also be described by negative numbers. For example, an object thrown toward the sky could have a positive velocity as it travels upward and a negative velocity as it travels down again.^[2] Consequently, the formula for speed and that for velocity also differ.

·         Acceleration is the rate at which an object is increasing in speed. It is described as a unit of distance (meters, kilometers, feet, miles, etc.) divided by a unit of time (usually seconds) squared. One of the most commonly used numbers in physics is 9.8 m/s(2). It represents the rate at which objects accelerate toward earth due to gravity.

2.Learn the difference between average and instantaneous speed. 

Instantaneous speed is the speed an object is traveling at any one moment. In a car, the speed reading on the speedometer indicates your instantaneous speed--the rate you are traveling at that particular moment. However, your speed for one particular instant does not have a direct impact on the average speed you travel. If you are traveling quickly for one second, but then slow down substantially after that, the brief burst of speed will make little difference on your total travel time. Average speed pertains to your speed between two set points. It is directly related to the time expended to get from the first point to the second.

3.Get to know the formula. 

The terms used to describe speed actually help explain the formula used to calculate it. Speeds of vehicles, for example, are typically described in miles per hour (mph, or m/h) or kilometers per hour (kph, or k/h). The term "per" is used to explain a rate. "Rates" are fractional terms--that is, they can be described as a fraction. Think of 50 miles per hour as a fraction: 50 miles/1 hour. The preferred terms for speed use divisors (in this case, units of time) that are a power of 1. From this, you can surmise the formula: speed = distance/time.

Part2

Applying What You Have Learned

1.Measure the distance traveled.

 As noted above, you can only determine average speed if you now the distance traveled by an object.

·         For practical applications such as travel by car, you can use online tools to determine the actual distance traveled along roads. Google Maps (maps.google.com), Mapquest (mapquest.com), and Michelin (viamichelin.com) all provide route information, including total mileage

2.Determine the amount of time taken to travel that distance.

The formal physics equation for speed is usually depicted as d/Δt, where d is distance and t is time.^[8]The Δ symbol (the classical Greek letter delta) represents change. You need to determine the change in time, from the starting point to the end point. If you only noted the time using a clock, you can do this by subtracting the start time from the finish time. (This is easiest if you convert all numbers to military time.)

·         If you began a trip at 8:00 am and finished at 1:00 pm, you would subtract 8:00 from 13:00. The trip took 5 hours.

3.Divide the distance by the time. 

Of course, the units used to describe time and distance are separate, and cannot cancel each other out during division. Any result will remain a ratio of distance to time.

·         As an example, let's say that you drove 150 miles in 3 hours. To determine the average speed, you would divide 150 miles by 3 hours. The result is 50 miles per hour (150m / 3hr= 50m/hr).