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Physical Science: Middle School9 chapters | 60 lessons

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Lesson Transcript

Instructor:
*David Wood*

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After watching this video, you will be able to explain what distance is, explain how it relates to change in position, and calculate the average speed in various situations. A short quiz will follow.

**Distance** is the amount of space between two things or people, or the amount something has moved, measured as a continuous line. We measure distance in units like miles, kilometers, meters, centimeters, inches, and yards. Sometimes it makes sense to use miles, like when you're driving in a car. And other times, it makes more sense to use centimeters, like measuring the distance between two objects on your desk. Miles are huge and centimeters are much smaller, so miles make sense on larger scales than centimeters.

Distance traveled is related to your change in position. Your position is where you are at a particular moment, and if your position changes by five meters, then you have traveled at least five meters. But distance is not the same thing as change in position. Here's why.

Let's imagine you're feeling really lethargic and decide you need to get moving. So, you get up from your desk and run around your house randomly, in and out of all the furniture. Then, you get back to your desk and sit down. What is your change in position?

Well, you're back where you started; so your change in position is zero. But you definitely traveled a distance. Your distance wasn't zero; you just ran all over the house. If you drew a line showing the path you took around the house, straightened out the line, and then measured it, that would be your distance. So, distance and change in position are not always the same number.

One thing you can do with distance is use it to figure out your average speed. **Speed** is the rate at which your distance changes, measured in meters per second or miles per hour or similar. Your speed tells you how many meters you travel each second, or how many miles you travel each hour. A speed of 50 miles per hour means that your distance is changing at a rate that would mean you traveled 50 miles every hour. 75 miles per hour is faster, for example, than 50 miles an hour, because at 75 miles per hour you would travel 75 miles every hour.

Okay, so that's speed. But what if your speed is changing all the time? You're driving a car and you speed up, and slow down, and speed up again. In that case, it might be more useful to figure out your average speed. The average speed you travel is given by this equation:

It's equal to the total distance you traveled, divided by the total time it took to travel that distance.

Now it's time to practice using the equation.

A car travels 50 miles over a period of two hours. What was the average speed of the car during those two hours?

Well, first of all let's write down what we know. The total distance is 50 miles, and the total time is 2 hours. Divide the distance by time, and we get 25. So the answer is that the average speed is 25 miles per hour.

A boy is riding a bicycle. He first travels two miles in 10 minutes, then he travels three miles in 20 minutes. What is the boy's average speed?

For this one, we need to add some things up. The total distance traveled is 2 miles plus 3 miles, which equals 5 miles. And the total time is equal to 10 minutes plus 20 minutes, which equals 30 minutes. But speeds are usually measured in miles per hour, not miles per minute, so instead of using 30 minutes, let's call it 0.5 hours, which is half an hour.

Plug the numbers into the equation. Divide distance by time, 5 divided by 0.5, and we get an average speed of 10 miles per hour.

**Distance** is the amount of space between two things or people, measured as a continuous line. We measure distance in units like miles, kilometers, meters, centimeters, inches, and yards. Distance traveled is related to your change in position, but it is not the same thing as change in position. Your position is where you are at a particular moment, and if your position changes by five meters, then you have traveled at least five meters. But if you run around your house and end up back where you started, your change in position is zero. But your distance traveled is definitely not zero.

One thing you can do with distance is to use it to figure out your average speed. **Speed** is the rate at which your distance changes, measured in meters per second or miles per hour. Your speed tells you how many meters you travel each second, or how many miles you travel each hour. But if your speed is changing all the time, it might be more useful to figure out your average speed. The average speed you're traveling is given by this equation:

It's equal to the total distance you traveled, divided by the total time it took to travel that distance.

After you've completed this lesson, you should be able to:

- Define distance and speed
- Identify common units of measurement for both distance and speed
- Differentiate between distance and change in position
- Explain how to calculate average speed

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Physical Science: Middle School9 chapters | 60 lessons

- What is Position in Physics? - Definition & Examples 4:42
- Speed and Velocity: Concepts and Formulas 6:44
- Distance, Time & Average Speed: Practice Problems 4:54
- Force: Definition and Types 7:02
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- Newton's Second Law of Motion: The Relationship Between Force and Acceleration 8:04
- Newton's Third Law of Motion: Examples of the Relationship Between Two Forces 4:24
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