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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 position is, why it is a vector, and interpret position-time graphs. A short quiz will follow.

Where are you right now? How many ways can you answer that question? Maybe you said, 'I'm in front of my computer.' Or maybe you said, 'I'm at home.' If you like to be more precise, you could even give your coordinates on the surface of the Earth. There are lots of ways of measuring where you are.

**Position** is a place where someone or something is located or has been put. In physics, position is usually a number on an axis. You can have an x-axis that looks like this. Or a y-axis, like this. If you drew these axes on the floor of a room, and then moved to a random place in that room, you would have a position in both the x and y directions. You might be at positive 6 meters in the x-direction, and negative 3 meters in the y-direction. That is your position.

When it comes to position, direction is important. By saying you're at positive 6 meters in the x-direction, you're saying that you are 3 meters to the right of the y-axis. This is a direction.

A number where direction matters is called a **vector**. A number where direction doesn't matter is called a **scalar**. For example, temperature is a scalar. Because it never has a direction. The temperature might be 70 degrees F, but it isn't 70 degrees F going up. It might be 40 degrees C, but never 40 degrees C headed west. That's not how temperature works. Temperature is a scalar.

Position is a vector because direction matters. But distance is a scalar. **Distance** is how far you've traveled. For example, if you run around your room with the axis on the floor, you might run a really long way, until you've gone a total of 50 meters. But your position isn't 50 meters. Your position might be negative 3 meters on the x-axis, and positive 4 meters on the y-axis, for example. The direction you ran made no difference to your distance - you still traveled 50 meters. So with distance, direction doesn't matter. Distance is a scalar.

When your position changes over time, you can show this using a position-time graph. A position-time graph looks like this, with position on the y-axis, and time on the x-axis. If you're not moving, you'll get a flat line, like this. If you're not moving at the origin, it will look like this. But if you're not moving at a position that isn't the origin, it will look like this.

If you're moving at a constant speed, you'll get a diagonal line. If you're moving towards the origin, it will look like this. If you're moving away from the origin, it will look like this.

You need to be able to plot a position-time graph from some data. For example, you might get some data like this. This data shows how a bicyclist changed position over the course of a day, going to work and then back home again. Pause the video and try plotting a graph of this data. I'll show you the solution in a moment.

If you haven't already, pause the video now.

Okay, if you're still listening to the video, you've had a go at plotting the graph. Now I'll show you what it should have looked like. Your graph should look like this. The bicyclist went to work at a constant velocity, stayed at the same position for a while because he was at work, and then cycled home. You might even notice that he cycled to work faster than he cycled home. Maybe he was late!

**Position** is a place where someone or something is located or has been put. In physics, position is usually a number on an axis. You can have an x-axis that looks like this. Or a y-axis, like this. You might be at positive 6 meters in the x-direction, and negative 3 meters in the y-direction. That is your position.

When it comes to position, direction is important. By saying you're at positive 6 meters in the x-direction, you're saying that you are 6 meters to the right of the y-axis. This is a direction. A number where direction matters is called a **vector**. A number where direction doesn't matter is called a **scalar**. Position is a vector, because direction matters. But distance is a scalar. **Distance** is how far you've traveled. For example, if you run around your room with the axes on the floor, you might run a really long way, until you've gone a total of 50 meters. But you position isn't 50 meters. The direction you ran made no difference to your distance - you still traveled 50 meters. So, with distance, direction doesn't matter. Distance is a scalar.

When your position changes over time, you can show this using a position-time graph. A position-time graph looks like this, with position on the y-axis, and time on the x-axis. If you're not moving, you'll get a flat line, like this. If you're moving at a constant speed, you'll get a diagonal line.

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Physical Science: Middle School9 chapters | 60 lessons

- What is Position in Physics? - Definition & Examples 4:42
- Distance, Time & Average Speed: Practice Problems 4:54
- Graphing Position & Speed vs Time: Practice Problems 5:05
- Force: Definition and Types 7:02
- Objects with Two or More Forces: Finding the Total Force Result 4:58
- Newton's First Law of Motion: Examples of the Effect of Force on Motion 8:25
- 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 6:05
- Forces: Balanced and Unbalanced 5:50
- Newton's Laws and Weight, Mass & Gravity 8:14
- Gravity in the Solar System: Shaping Planets & Stars 4:58
- Go to Motion & Forces

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