Back To Course

Physics 111: Physics I16 chapters | 140 lessons | 11 flashcard sets

Are you a student or a teacher?

Try Study.com, risk-free

As a member, you'll also get unlimited access to over 75,000 lessons in math, English, science, history, and more. Plus, get practice tests, quizzes, and personalized coaching to help you succeed.

Try it risk-freeWhat teachers are saying about Study.com

Already registered? Log in here for access

Your next lesson will play in
10 seconds

Lesson Transcript

Instructor:
*Betsy Chesnutt*

Betsy teaches college physics, biology, and engineering and has a Ph.D. in Biomedical Engineering

What is the difference between average and instantaneous velocity? How do you calculate each of these and when do you use them? This lesson will help you understand these common terms and know when to use each one.

Jack was on his way to school when he had to stop his car to let a train pass. At that point, he had already driven three miles from his house, and he still had five more miles to go to reach his school. As he sat in his car waiting, Jack tried to calculate how fast he would need to drive to still make it to school on time.

He knew that he needed to calculate his velocity, but he wasn't really sure how to do that. He remembered from his physics class that **velocity** was defined as the rate of change of an object's position.

However, to calculate his velocity, he didn't know if he should use the total distance he was traveling to school (eight miles), the distance he had already driven (three miles), or the distance he still had to go (five miles). Also, what about the fact that he was now stopped waiting on the train? Wasn't his current velocity zero since his car wasn't moving?

Wow! Determining the velocity of an object isn't as simple as Jack thought at first. What can he do?

The issue Jack is facing is that there are two different ways that velocity can be calculated, and they each give you very different information about an object's motion. The **average velocity** of an object is found by dividing its total displacement by the total time it took to move from one place to another.

**average velocity = change in position / change in time = d / t**

Average velocity doesn't tell you exactly how fast an object is moving at every instant in time. The object can speed up, slow down, and even stop for a while, and this will not directly affect its average velocity. If it took Jack a total of 30 minutes to travel the eight miles from his house to school, what was his average velocity?

**Vav = d / t = (8 miles ) / (30 minutes) = 0.27 miles/min**

In contrast, **instantaneous velocity** is the velocity of the object at a single instant in time. This may or may not be the same as the average velocity over a longer time interval. In Jack's case, while he is sitting still waiting for the train to pass, his instantaneous velocity is zero because his car is not currently moving at all. However, his average velocity on the entire trip to school will not be zero even though the instantaneous velocity was zero for a small part of the trip.

If you represent the position of an object over time on a graph, you can use the graph to find both the average and instantaneous velocity of the object at any time. Let's look at a position versus time graph of Jack's entire trip to school to see how this works:

In the first six minutes, Jack went three miles. Then, he stopped and waited on the train for nine more minutes, and finally, he traveled the remaining five miles to school in fifteen minutes.

We can find Jack's average velocity from the graph by drawing a line between the beginning point and the ending point and then finding the slope of that line.

To find his instantaneous velocity at a certain point in time, we can find the slope of just that part of the graph. In this case, Jack had three different velocities during the course of his trip. In the first six minutes, his instantaneous velocity was 0.5 miles/minute. Then, while he was stopped, his instantaneous velocity was 0 miles/minute. Finally, during the last fifteen minutes, his instantaneous velocity was 0.33 miles/minute. None of these instantaneous velocities was the same as the average velocity for the entire trip!

There are some cases when the average velocity and instantaneous velocity of an object will be the same. If an object's velocity is not changing, we say that it has a **constant velocity**. Whenever an object has a constant velocity, the average velocity will be the same as the instantaneous velocity. An example of this is a car driving down the highway at a constant speed of 60 miles per hour. The average velocity of the car is 60 miles per hour, and no matter what time you measure the instantaneous velocity, it will always be 60 miles per hour.

Average velocity and instantaneous velocity each provide you with different information about the motion of an object, and there are times when one may be more useful than the other. For example, cars have a speedometer that shows your instantaneous velocity. It's a good idea to keep track of that so that you don't exceed the speed limit and get a ticket! If you are speeding and get pulled over by a police officer, he won't care about your average velocity, but only about the instantaneous velocity of your car!

However, if you want to know how long it will take to get somewhere, average velocity might be more helpful. If you know that you can drive at an average velocity of 1 mile/minute, then you can determine how long it will take you to travel a certain distance. It doesn't matter if you are going at 1 mile/minute the whole time, as long as your average velocity is 1 mile/minute.

The **velocity** of an object tells you how quickly it's moving, and velocity is formally defined as the rate of change of an object's position. The **average velocity** of an object is found by dividing the total displacement of the object by the total time it took to move from one place to another.

**average velocity = change in position / change in time = d / t**

On a graph of position vs. time, average velocity can be found by drawing a line between two points on the graph and finding the slope.

**Instantaneous velocity** tells you the velocity of an object at a single moment in time. If the object is moving with a **constant velocity**, then the average velocity and instantaneous velocity will be the same. In all situations, they are not likely to be the same.

To unlock this lesson you must be a Study.com Member.

Create your account

Are you a student or a teacher?

Already a member? Log In

BackWhat teachers are saying about Study.com

Already registered? Log in here for access

Did you know… We have over 160 college courses that prepare you to earn credit by exam that is accepted by over 1,500 colleges and universities. You can test out of the first two years of college and save thousands off your degree. Anyone can earn credit-by-exam regardless of age or education level.

To learn more, visit our Earning Credit Page

Not sure what college you want to attend yet? Study.com has thousands of articles about every imaginable degree, area of study and career path that can help you find the school that's right for you.

You are viewing lesson
Lesson
8 in chapter 4 of the course:

Back To Course

Physics 111: Physics I16 chapters | 140 lessons | 11 flashcard sets

- Distance and Displacement in Physics: Definition and Examples 5:26
- Graphing the Motion of Objects: Physics Lab 3:17
- Using Position vs. Time Graphs to Describe Motion 4:35
- Speed and Velocity: Difference and Examples 7:31
- Determining Slope for Position vs. Time Graphs 6:48
- Using Velocity vs. Time Graphs to Describe Motion 4:52
- Velocity vs. Time: Determining Displacement of an Object 4:22
- Average vs. Instantaneous Velocity: Difference & Uses 5:40
- Determining Acceleration Using the Slope of a Velocity vs. Time Graph 5:07
- Understanding Graphs of Motion: Giving Qualitative Descriptions 5:35
- Overview of Graphs of Free Fall Motion 4:53
- Go to Displacement, Velocity & Acceleration

- SIE Exam Study Guide
- Indiana Real Estate Broker Exam Study Guide
- Grammar & Sentence Structure Lesson Plans
- Foundations of Science Lesson Plans
- Career, Life, & Technical Skills Lesson Plans
- Business Costs, Taxes & Inventory Valuations
- Using Math for Financial Analysis
- Assessments in Health Education Programs
- Governmental Health Regulations
- Understanding Health Education Programs
- AFOQT Prep Product Comparison
- ACT Prep Product Comparison
- CGAP Prep Product Comparison
- CPCE Prep Product Comparison
- CCXP Prep Product Comparison
- CNE Prep Product Comparison
- IAAP CAP Prep Product Comparison

- What is Deadlock? - Definition, Examples & Avoidance
- El Hombre que se Convirtio en Perro: Author, Summary & Theme
- Achilles in The Iliad: Character Analysis & Description
- A Wrinkle in Time Chapter 5 Summary
- Roald Dahl Project Ideas
- Media Literacy Activities for High School
- Letter M Activities
- Quiz & Worksheet - Shang Dynasty Religion & Culture
- Quiz & Worksheet - Alternative Assessment Types
- Quiz & Worksheet - Population Composition
- Quiz & Worksheet - Minimalist Painters
- Analytical & Non-Euclidean Geometry Flashcards
- Flashcards - Measurement & Experimental Design
- Life Skills Resources for Teachers
- Middle School Science Worksheets and Printables

- Holt Science Spectrum - Physical Science: Online Textbook Help
- Glencoe Geometry: Online Textbook Help
- NY Regents Exam - Earth Science: Help and Review
- Biology 105: Anatomy & Physiology
- History 101: Western Civilization I
- GRE - Equations and Expressions: Help and Review
- How to Cite Source Materials: Homeschool Curriculum
- Quiz & Worksheet - Types of Irrigation
- Quiz & Worksheet - The Cenozoic Era
- Quiz & Worksheet - Supply Chain Integration
- Quiz & Worksheet - Atmosphere in Literature

- What is a Proportion in Math? - Definition & Practice Problems
- What is the Pineal Gland?
- PSAT Test Dates
- Kentucky Science Standards
- Earth Science Projects
- Typical Medical School Curriculum
- PSAT Tips & Tricks
- Best Study Abroad Programs
- Reading Comprehension Lesson Plan
- The Highwayman Lesson Plan
- Poetry Lesson Plan
- Frindle Lesson Plan

- Tech and Engineering - Videos
- Tech and Engineering - Quizzes
- Tech and Engineering - Questions & Answers

Browse by subject