AVERAGE SPEED: HOW TO CALCULATE IT AND EXAMPLES - PHYSICAL

The average speed or average speed is defined as the quotient between the space traveled and the time used to travel that space. Speed is a fundamental quantity both in physics and in people's daily lives. It is present in almost every aspect of people's lives.

This presence of speed is especially noticeable in today's society, where there is a growing demand for immediacy. Of course, speed is also intrinsically related to a multitude of physical phenomena. Somehow, all people have an intuitive idea, more or less correct, about the concept of speed.

It is necessary to distinguish between average speed and instantaneous speed. The instantaneous speed is the speed that a body is carrying at a given instant, while the average speed is the quotient between the displacement and the time.

Furthermore, it should be noted that velocity is a scalar quantity; that is, it has a direction, a sense, and a module. In this way, speed is applied over one direction.

In the international system, speed is measured in meters per second (m / s), although other units are often used in everyday life, such as kilometers per hour (km / h).

How to calculate it?

The calculation of the average speed is carried out from the following expression:

v _m = ∆s / ∆t = (s _{f -} s ₀) / (t _{f -} t ₀)

In this equation v _m is the mean velocity, ∆s is the displacement increment and ∆t is the time increment. On the other hand, s _f and s ₀ are the final and initial displacement, respectively; while t _f and t ₀ are the final and initial time, respectively.

Another expression for calculating the average speed is:

v _m = s _t / t _t

In this expression s _t is the total displacement and _{t t} is the total time invested in making that displacement.

As can be seen in this calculation, only the total displacement and the total time used in it are taken into account, without at any time it being necessary to take into account how this displacement occurred.

Nor is it necessary to know if the body has accelerated, stopped, or traveled all the way at a constant speed.

Often it may be necessary to do the reverse calculation to determine the total displacement from the average speed and the total time taken.

In that case, you just have to solve for the displacement of the first equation to obtain the expression that allows it to be calculated:

∆s = v _m ∙ ∆t

The same could be done if it is necessary to calculate the time used in a movement carried out at a known average speed:

∆t = v _m ∙ ∆s

Units of measure for speed

Speed can be expressed in different units. As mentioned above, in the International System the unit of measurement is the meter per second.

However, depending on the context, it may be more convenient or more practical to use other units. Thus, in the case of means of transport, the kilometer per hour is usually used.

For its part, in the Anglo-Saxon System of Units they use the foot per second (ft / s) or the mile per hour (mph) in the case of means of transport.

In maritime navigation the knot is commonly used; On the other hand, in aeronautics the Mach number is sometimes used, which is defined as the quotient between the speed of a body and the speed of sound.

Examples of calculating average speed

First example

A typical example where it may be necessary to calculate the average speed is a trip between two separate cities.

Suppose the case in which both the total displacement (which does not have to coincide with the distance between the two cities) made on the journey between the two cities - for example, 216 kilometers - as well as the time spent on that journey is known -for example, three hours-.

The calculation of the average speed would be done like this:

v _m = ∆s / ∆t = 216/3 = 72 km / h

If you want to express the velocity in International System units, you should carry out the following conversion:

v _m = 72 km / h = 72 ∙ 1000/3600 = 20 m / s, given that a kilometer is a thousand meters and an hour has 3600 seconds.

Second example

Another practical case of calculating the average speed is when several movements have been made in a given period of time.

Suppose a woman who has made several trips by bicycle over several days and wants to know what the average total speed of her trip has been.

The woman has traveled the following distances throughout the successive days: 30 kilometers, 50 kilometers, 40 kilometers and 20 kilometers.

The respective times used have been the following: an hour and a half, two hours and a half, 2 hours and a half, and an hour and a half. Then the resulting average speed is calculated as follows:

v _m = (30 + 50 + 40 + 20) / (1.5 + 2.5 + 2.5 + 1.5) = 17.5 km / h

Examples of average speed

It may be interesting to know some examples of average travel speeds to have a more intuitive idea of the different values that the speed can take.

In the case of a person walking, the value of his average speed is considered to be 5 kilometers per hour. If that same person runs, she can reach half that average speed.

The average speed of an amateur cyclist can be estimated at around 16 kilometers per hour, while for a professional cyclist on the road the average speed reaches the value of 45 kilometers per hour.

Category 1 hurricanes can have an average speed of 119 kilometers per hour. Finally, the average orbital speed of the Earth around the Sun is 107,218 kilometers per hour.

References

Speed (nd). In Wikipedia. Retrieved on April 23, 2018, from en.wikipedia.org.
Speed (nd). In Wikipedia. Retrieved on April 23, 2018, from es.wikipedia.org.
Kilometer per hour (na). In Wikipedia. Retrieved on April 23, 2018, from es.wikipedia.org.
Richard P. Feynman, Robert B. Leighton, Matthew Sands. The Feynman Lectures on Physics.
Elert, Glenn. "Speed & Velocity". The Physics Hypertextbook. Retrieved on April 23, 2018.

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