# Classical Relativity: Distance and Time Relation to the Observer

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• 0:05 Classical Relativity:…
• 0:57 Classical Relativity:…
• 2:22 Everything Moves
• 2:51 Complex Example
• 4:45 Exception to the Rule
• 5:24 Lesson Summary
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Lesson Transcript
Instructor: John Simmons

John has taught college science courses face-to-face and online since 1994 and has a doctorate in physiology.

It is not enough to ask how fast something is moving. We must ask how fast something is moving relative to something else. This lesson describes how classical relativity is used to explain how speed is relative to the state of motion of the object and the observer. Examples are used to help understand classical relativity.

## Classical Relativity: Distance and Time

Relativity is just a way for two people to agree on what they see from different perspectives. One of the most obvious examples of relativity is our perception of relative distance. Buildings appear smaller when they are farther away. Even from the same distance, objects can appear different depending on the perspective. Looking at a house from the corner makes the sides seem to be shorter than if we look at the same house straight on. Relativity allows us to agree on the size of the building regardless of the distance or the perspective from which they are observed.

Relativity gets a little more complex when motion is involved. Classical relativity provides a way for two people to agree on what they see if one of those people is moving.

## Classical Relativity: Speed and Motion

Two people can observe the same event as being different depending on their perspective. Imagine going for a drive with your best friend. It's a long drive so be sure to bring a thermos of coffee. If you are driving at a constant velocity of 65 mph straight down a stretch of highway, you can easily pour a cup of coffee. Now this is possible because the car, you, the thermos, the coffee, and your cup are all traveling at a constant velocity. From the perspective of you and your friend, the coffee is not moving and thus appears to have a velocity of 0 mph.

If someone were watching you drive by, he would observe the coffee moving at a velocity of 65 mph - that is, the same velocity of the car. Both perspectives are correct. The coffee is moving at 0 mph relative to the car and 65 mph relative to the road. As 65 mph is not the same as 0 mph, it is not enough to ask how fast something is moving. We must ask how fast an object is moving relative to something else. In other words, motion is relative.

## Everything Moves

If you are viewing this video while sitting or standing still, you're traveling at 0 mph relative to the ground. However, the earth is spinning on its axis at 1000 mph. Additionally, the earth is rotating around the sun, and the sun is moving around our galaxy. That's a lot of motion. To say that you are not moving when sitting still is accurate but only relative to the ground.

## Complex Example

Let's look at a bit more complex situation. Imagine playing catch with your best friend on the back of a truck traveling at 65 mph down that same straight stretch of highway. Bring your coach along and have him measure the velocity of the ball using a radar gun. As you play catch, your coach clocks you throwing the ball at 50 mph. Your coach then clocks your friend throwing the ball at 40 mph. What do these speeds mean in terms of relativity? You threw the ball with a velocity of 50 mph relative to the truck, and your friend threw the ball with a velocity of 40 mph relative to the truck.

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