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TEKS 8.6B Differentiate between speed, velocity, and acceleration.

TEKS Lesson 8.6B:

Speed, Velocity, and Acceleration

What is Speed?

You might describe the motion of an airplane as fast or the motion of a snail as slow. By using these words, you are describing the object’s speed. The speed of an object is the distance the object moves per unit of time.

To calculate the speed of an object, divide the distance the object travels by the amount of time it takes to travel that distance. This relationship can be written as an equation.

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If you measure distance in meters and time in seconds, the SI unit for speed is meters per second, or m/s. For example, at its cruising altitude, an airplane might travel at a constant speed of 260 m/s, meaning that the airplane will travel a distance of 260 meters in

1 second. Other units of distance include kilometers (km) and centimeters (cm). Another unit of time often used in measuring speed is hours (h). A typical speed limit for cars on city streets is about 40 km/h.

1. Define What two quantities do you need to know to measure speed?

What is Velocity?

Knowing the speed at which something travels does not tell you everything about its motion. To describe an object’s motion, you also need to know its direction. Together, the speed and direction in which an object is moving are called velocity. Velocity is speed in a given direction.

Suppose you hear that a thunderstorm is traveling at a speed of 25 km/h. Should you prepare for the storm? You won’t know the answer until you also know the storm’s direction of motion. If the storm is moving to the east and you live to the north of the storm, you do not need to worry.

If either the speed or the direction of motion of an object changes, its velocity changes. For example, if the storm described above continues eastward but slows to 20 km/h, its velocity has changed. On the other hand, its velocity also changes if the storm continues to move at 25 km/h but shifts toward the north. And in that case, the storm is headed

towards you.

2. Review What two kinds of information do you need to describe the velocity of an object?

What is Acceleration?

Suppose you are a passenger in a car stopped at a red light. When the light changes to green, the driver steps on the accelerator. As a result, the car speeds up, or accelerates. In everyday language, acceleration means “the process of speeding up.” However the word has a more precise definition in science.

Acceleration is the rate at which velocity changes. A change in velocity can involve a change in either speed or direction—or both. In science, acceleration refers to increasing speed, decreasing speed, or changing direction.

Changing Speed Whenever an object’s speed changes, the object accelerates. For example, a car that speeds up to pass another car is accelerating. You accelerate if you coast down a hill on your bike.

Just as objects can speed up, they can also slow down. This change is speed is sometimes called deceleration, or negative acceleration. For example, a car decelerates as it comes to a stop at a red light. A water skier decelerates as the boat slows down.

Changing Direction Even an object that is traveling at a constant speed can be accelerating. For example, a car accelerates as it follows a gentle curve in the road or if it changes lanes. Many objects continuously change direction without changing speed. The simplest example of this type of motion is circular motion, or motion along a circular path. For example, the seats on a Ferris wheel accelerate because they move in a circle even though they are moving at a constant speed.

3. Describe In what ways can acceleration of an object occur?

How Can You Differentiate Between Speed, Velocity, and Acceleration?

You have learned about three properties of motion. Speed is a measure of distance traveled per unit of time. When you describe the speed and the direction of motion of an object, you are describing the object’s velocity. If you describe a change in the object’s speed or in its direction of motion, you are describing acceleration.

4. Describe What does velocity tell you about an object that speed

does not?

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Lesson Check

1. Identify Which of the following is a correct way to report an example of the speed of an object?

A 50 s/m B 50 m/km

C 50 m/s D 50 s/km

2. Distinguish How are speed and velocity different?

3. Define What describes the rate at which velocity changes?

4. Differentiate Use the sentences below to differentiate between speed, velocity, and acceleration.

A bus that is traveling north at 48 km/h slows to a stop to pick up passengers. It then begins moving again. At the next corner, it turns to the east.

a. Circle the speed of the bus.

b. Underline one example of the velocity of the bus.

c. Draw two underlines below each group of words or phrases that describe acceleration of the bus.

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TEKS

Grade

Lesson 8.6B

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