# What Speed Really Means

Contributor: Jay Gregorio. Lesson ID: 13206

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

categories

## Physics

subject
Science
learning style
Kinesthetic, Visual
personality style
Beaver
High School (9-12)
Lesson Type
Dig Deeper

## Lesson Plan - Get It!

Audio:

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, a few names set themselves apart from other athletes because of their incredible speed.

Usain Bolt is one such athlete.

Sports fanatics worldwide followed as the former champion Usain Bolt beat his record repeatedly. In the 2009 Berlin World Championships, he ran 100 meters in 9.58 seconds.

• Can you imagine how fast he is?

Watch a section of the video below for his 9.58-second world-record-breaking run in 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 the earth!

The concept of speed involves two different fundamental quantities: length and time.

In this case, the length, also called distance, 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 covers when you trace it from the beginning to the end. Considering that an object does not necessarily travel in straight lines, the strategy for determining the distance can vary.

Take a look at the following examples.

Figure 1

Figure 1 is the most simple way to calculate distance. This is an example of an object traveling in one dimension, either along the horizontal or vertical lines.

Using a measuring tool, such as a meter stick, you simply place the zero point of the stick at point A and then determine where point B ends.

In this example, the distance is 2 meters or 2 m.

Figure 2

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 points A to B, then add it to the length of points B to C. This measurement is the total distance.

In this example, the total distance is 4 meters or 4 m.

Figure 3

Figure 3 involves a challenging distance calculation involving many curves or turns.

You may think you can use a string to trace the path's shape and straighten the string to get the actual length. While you can use this strategy for very short distances, it is not practical if you are talking about miles of distance.

A special mathematical process called calculus deals with these types of problems.

This lesson will focus only on the situations shown in Figures 1 and 2.

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 occurring during a given period.

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 minimal 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.

• 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, it looks like this.

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 covers 2 m to the right and 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 travel at the same speed because it had to change direction.

Say it took the object 2 s to complete this trip. Therefore, you can say that the average speed of the object is the total distance covered divided by time.

In expression form, it looks like this.

Average Speed = Total Distance / Time

vaverage = total / 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, practice calculating the speed of an object! Get ready!

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