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High School Physical Science: Help and Review32 chapters | 347 lessons

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Lesson Transcript

Instructor:
*Amanda Robb*

In this lesson, we'll learn about instantaneous speed, the difference between it and average speed, and the formulas for calculating both of them. Examples will be provided, and a quiz will let you test your understanding.

As you're sprinting on the track getting your daily exercise, a plane zooms overhead. Cars fill the street, some speeding along, other stuck in traffic. Your friends wander over to the track, waiting for you to finish exercising so you can take the bus home together. What do all these daily occurrences have in common? The answer is speed.

**Speed** is how fast something is moving, and things in the world are frequently moving. In fact, Earth itself is always moving, although this isn't apparent in our daily lives.

There are two ways we can measure how fast something's moving:

- By calculating instantaneous speed
- By calculating average speed

Let's explore how these two measurements are different.

When a cop pulls you over for speeding, she clocked your car's **instantaneous speed**, or speed at a specific point in time as your car sped down the road. 'Instantaneous' comes from the word 'instant' meaning only one specific moment.

This is different than your **average speed** from the trip, which takes into account how long it took to complete the entire journey and the distance traveled. Be careful, measuring average speed assumes you moved at about the same speed the entire trip.

To calculate instantaneous speed, we need to divide part of the total distance traveled by time. However, we don't want to use the distance of the entire trip, because that will give us average speed. We use a small distance of the trip. The smaller the distance used, the more accurately we can measure the speed for that specific time.

Let's look at some examples to make this concept more clear.

Let's suppose it's a snowy day. You're in a Boeing jet at a New York City airport, taking a trip to Chicago. Your plane is taxiing down the runway when it suddenly stops. There will be a delay as the plane is de-iced for air travel. This adds an extra hour onto your journey.

Normally this flight would take about two hours. However, now the trip's total time is three hours due to the delay. Since average speed takes into account the entire time of the trip, which is approximately 1,200 kilometers, the average speed of the plane is 400 kilometers per hour.

However, when a Boeing jet is traveling at cruising altitude, the instantaneous speed is about 800 km/hr based on measuring a distance of 80 km in 0.1 hr. The plane would be traveling far too slow if the average speed was the same as the instantaneous speed of the plane in the sky.

Okay, let's say that you're riding your bike home. Your instantaneous speed varies depending on where you are in your trip. Starting out from a red light, you are going very slow. However, cruising downhill you might reach nearly 20 miles per hour. Likewise, going back up that hill, your speed might slow to a crawl.

The average speed would take into account the total distance divided by the total time it took you to complete the trip, but your instantaneous speed would vary depending on where you are.

In summary, **instantaneous speed** is the speed of an object at any particular moment in time. It is different from **average speed** because average speed is measured by the total time of a journey divided by the total distance. In contrast, instantaneous speed measures the smallest interval possible divided by the time it took to move that distance.

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High School Physical Science: Help and Review32 chapters | 347 lessons

- What Is Motion? - Definition & Equations 5:56
- Speed and Velocity: Concepts and Formulas 6:44
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- Implications of Mechanics on Objects 6:53
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