*Contributor: Jay Gregorio. Lesson ID: 13206*

A former Jamaican sprinter named Usain Bolt is considered the fastest man on earth, but just how fast is he? What does fast even mean? Let's find out what speed really is and how it is calculated!

categories

subject

Science

learning style

Kinesthetic, Visual

personality style

Beaver

Grade Level

High School (9-12)

Lesson Type

Dig Deeper

One of the most popular and prestigious events in athletics is the 100-meter sprint race in the track and field competition. In the Olympics for instance, there are a few names that set themselves apart from other athletes because of their incredible speed. Usain Bolt is one such athlete.

Sports fanatics from all over the world followed as the former champion Usain Bolt beat his record over and over again. In the 2009 Berlin World Championships, he ran 100 meters in 9.58 seconds.

- Can you imagine how fast is he?

Watch a section of the World Athletics video below to see his 9.58 second world-record breaking run.

*World Record | Men's 100m Final | World Athletics Championships Berlin 2009*:

A cheetah can run this same distance in 5.8 seconds! That means Usain Bolt is only 3.78 seconds shy of a cheetah's running speed! Indeed, the fastest man on earth!

The concept of speed involves two different fundamental quantities - length and time. The length, also called distance in this case, describes *how far* an object travels. The time describes *how long* an object travels a particular distance. If both of these quantities are satisfied, it would be easier to determine *how fast* an object is moving.

**How Far?**

In physics, the distance traveled by a moving object is the measurement of the total path that the object covered when you trace it from the beginning to the end. Considering that an object does not necessarily travel in straight lines, the strategy in determining the distance can vary. Let us take a look at the following examples.

Figure 1 is the most simple way to calculate distance. This is an example of an object that is traveling in one dimension, either along the horizontal line or along the vertical line. Using a measuring tool, such as a meter stick, you simply place the zero point of the stick at point A then determine where point B ends. In this example, the distance is 2 meters or 2 m.

Figure 2 requires determining the *total distance* because the object is moving in two different directions, right then forward. In the same manner described above, you use the measuring tool to determine the length of point A to B then add it to the length of point B to C. This measurement is the *total distance*. In this example, the total distance is 4 meters or 4 m.

Figure 3 involves a challenging distance calculation because it involves many curves or turns. You may be thinking that you can use a string to trace the shape of the path and straighten the string to get the actual length. While you can use this strategy for very short distances, it is not practical to do so if you are talking about miles of distance. There is a special mathematical process called *calculus* that deals with these types of problems. In this lesson, we will focus on the situations shown in Figures 1 and 2 only.

**How Fast?**

The examples above gave you an understanding of how distance or total distance is measured. The rate at which a distance is covered is called *speed*. *Rate* is a word used to describe something that occurs at a given period of time. In short, anything divided by time is a rate. When you hear the phrase *the rate of distance covered*, they are actually saying *how fast* an object is moving.

There are different units of measure for speed depending on what moving object is being described. It would be impractical to state a cheetah's speed in millimeters per second or a snail's speed in miles per hour since that would give you a very large or a very small number, respectively. Examine the examples below:

- Usain Bolt's top speed is 27.8 miles per hour or 27 mph.
- A cheetah's speed can reach 96.5 kilometers per hour or kph.
- Tortoises can move at a beginning speed of 0.05 meters per second or 0.05 m/s.
- The Lockheed SR-71 Blackbird is the fastest jet ever developed, with a speed of 2,193.2 miles per hour or mph.

Learn more about speed in this video from Don't Memorise.

*What is Speed? - CBSE 7*:

- How do you calculate speed?

The mathematical expression for speed is straightforward. Speed is expressed as the distance divided by time:

Speed = Distance / Time

In symbols:

v = d / t

While there are many units of measure to show how fast an object is moving, the standard unit of speed is meters per second (m/s). Suppose that the object in Figure 1 covered 2 m in 0.5 s.

- How fast is the object moving?

v = d / t

v = 2 m / 0.5 s

v = 4 m/s

The video mentioned that we are actually calculating *average speed*. This concept is easier to understand in Figure 2 where the object covered 2 m to the right then 2 m forward. In your head, you know that the object traveled a total distance of 4 m. However, the object could not always be traveling at the same speed because it had to change direction.

Let us say that it took the object 2 s to complete this entire trip. Therefore, you can say that the average speed of the object is the total distance covered divided by time. In expression:

Average Speed = Total Distance / Time

vaverage = dtotal / t

vaverage = 4 m / 2 s

vaverage = 2 m/s

Simply put, if the object traveled multiple straight paths, you can add them up to find the total distance covered, then divide this number by the amount of time it took to complete the trip.

In the *Got It?* section, you will practice calculating the speed of an object! Get ready!