Hands-on Activity Exploring Center of Gravity in Motion Caption Technology

Quick Look

Grade Level: 6 (5-7)

Time Required: 2 hours 45 minutes

Plan on spending one or two 50-minute classes to introduce the concepts of motion capture, animation and action-writing followed by two or three 20-minute sessions to critique drafts of the written action sequence.

Expendable Cost/Group: US $0.00

Group Size: 1

Activity Dependency: None

Subject Areas: Physical Science, Physics

In simple lines representing a body and limbs, the essence of a figure moving across a grid is illustrated.
A computer graphic showing the animation of motion capture.
Copyright © Jovan Popović , Massachusetts Institute of Technology, http://graphics.lcs.mit.edu/~jovan/mocap.html. Used with permission.


Students learn how motion capture (mo-cap) technology enables computer animators to create realistic effects. They learn the importance of center of gravity in animation and how to use the concept of center of gravity in writing an action scene. Note: The literacy activities for the Mechanics unit are based on physical themes that have broad application to our experience in the world — concepts of rhythm, balance, spin, gravity, levity, inertia, momentum, friction, stress and tension.

Engineering Connection

Since people naturally have an intuitive sense about balance, computer animation designers who want to create realistic effects must convey a convincing sense of balance in their work Authentic-looking special effects, such as collapsing bridges or exploding structures, must first convey a loss in equilibrium before they fail. Engineers must also be convincing communicators. When writing to persuade, it is sometimes helpful to catch the reader off balance to make a dramatic point.

Learning Objectives

After this activity, students should be able to:

  • Use a full range of strategies to comprehend technical writing, newspapers, magazines, poetry, short stories, plays and novels.
  • Choose vocabulary and figures of speech that communicate clearly.
  • Learn the importance of center of gravity in animation and how to use the concept of center of gravity in writing an action scene.

Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

  • Explain how various relationships can exist between technology and engineering and other content areas. (Grades 3 - 5) More Details

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Materials List

  • Paper and pencils
  • Access to the Internet

Pre-Req Knowledge

General familiarity with the concepts of center of mass, center of gravity, balance and gravity


You have learned (in Mechanics unit, Lesson 6) that you and everything else on Earth have a center of gravity. Becoming aware of this concept can be very useful. Your balance might improve; you might get better at sports. You might also become a better writer.

It may surprise you to learn that sports writers are considered to be among the best writers in journalism. That's not really so surprising when you consider their subject — action. They literally write where the action is. Over time, they become very good at capturing the essence of action. Action is organized around a center — center of gravity — and that fact helps sports journalists to organize their writing. They also use lots of action verbs, short paragraphs, fast-paced description and fragment sentences.

A motion capture image demonstrating center of gravity.
Copyright © Image courtesy of Chi-Wei (Wayne) Chu, University of Southern California, http://www-scf.usc.edu/~chuc/volcap/. Used by permission.

Your teacher has probably been trying to get you to write complete sentences. That's good! You need to learn the rules in order to be in control of them. What you will find in this activity is that there is a time and a place for an occasional fragment sentence. That place is an action sequence.

In this activity, you will write an action sequence for computer animation. To prepare, you are going to learn about the latest techniques of motion capture, advanced technology that captures and digitizes movement data and applies it to a virtual actor in real time. It makes the "special effects" look realistic, primarily because the action of the figure is anchored in and organized around the actual movements of a human being whose motion is being captured. As an example, the Golum character in the Lord of the Rings film trilogy is a noteworthy achievement in motion capture.


In addition to being important for human equilibrium and balance, center of gravity can make the difference between convincing and unconvincing computer animation. The first Spiderman and The Incredible Hulk films were criticized for not achieving a convincing sense of real motion. Ironically, the motions of the characters were too gravity-defying. To be realistic, even high-flying movements have to convey a sense that gravity is still having an effect. Animated characters have to give the impression of having a center of gravity.

In this activity, students explore the concept of center of gravity by learning about the emerging field of motion-capture (mo-cap) in computer animation and special effects. They also learn how important the concept of a center of gravity is in hand-drawn action art and animation. Finally, they apply the concept of a "center of gravity" as a way to organize a written fantasy action sequence.


For background reading and research, see the following links or search the internet using keywords such as: motion capture, performance capture, optical motion capture, or motion (or performance or optical) capture video.

Carefully study Figure Drawing: Basic Pose and Construction by William Li, http://bhsartdepartment.weebly.com/uploads/6/5/9/6/6596506/figure_drawing.pdf. It includes an excellent analysis of the concept of center of gravity as related to figure drawing. The author makes the point, "To construct something you must understand it." This is as true of writing as it is of drawing. As a warm-up for writing your action sequence, you might want to experiment with the drawing techniques described. Your drawing ability is likely to improve along with your writing ability.


In this activity, the title of this section could also be named "Experiencing." Get in touch with your own center of gravity. That's one of the best ways to prepare to write an action scene. Act out the scene and think through the motions as they are organized around the center of gravity. Do the same when observing how mo-cap is executed.


Now, write your action sequence. The "Write with Fey" website provides an excellent guide, read "How to Write Action" available at http://writewithfey.blogspot.com/2012/06/how-to-write-action.html.


Center of gravity: The point at the center of an object's weight distribution.

Center of mass: The point at the center of an object's mass distribution. For objects on Earth, this is the same as the object's center of gravity.

Motion capture: A process by which external devices can be used to capture movement data from various live sources. This data is then transmitted to the computer, where 3D simulation software displays it in real-time applied to a virtual actor.

Virtual: Created, simulated or carried on by means of a computer or computer network.


Pre-Activity Assessment

Call-Out Questions: Quiz students with call-out questions during the Observing and Thinking phases to be sure they understand the concepts of motion capture, center of gravity and how animation works.

Activity Embedded Assessment

Writing Drafts: This activity works through a succession of drafts that can be evaluated individually.

Post-Activity Assessment

Expressive Writing: Success in capturing the feel of the action should be the primary criterion in evaluating the final draft of the action sequence. The use of fragment sentences is acceptable in this activity, if effectively used.

Discussion Questions: Ask the students, and discuss as a class.

  • What is the center of gravity? (Answer: The point at the center of an object's weight distribution, so that at this point the body is balanced [or in equilibrium] in all positions. For objects on Earth, this is the same as the center of mass, which is the point at the center of an object's mass distribution.)
  • Describe animation examples from movies or cartoons, in which you could tell that the center of gravity was or was not being portrayed realistically. (Answers: Vary with students' experience. For example, any cartoon in which the character has large, unbalanced features, like a nose that is so gigantic that his neck could never hold it up and it would probably be so heavy that it should drag on the ground. Or, a cartoon character that walks off a cliff and then spins his legs in the air to get back to the cliff, never falling!)
  • Suggest examples of design projects in which engineers take into consideration an object's center of gravity or center of mass. (Possible answers: Designing airplanes or helicopters [for stability in the air], bridges or buildings [so they will not fall over], golf clubs or chef's knives [for ideal balance], etc.)
  • Suggest examples in which working with your own center of gravity is important for success. (Possible answers: Riding a bicycle, diving into a pool, stepping on rocks to cross a stream, carrying a stack of dishes, sitting down, standing up, hopping on one foot, walking a balance beam or curb, etc.)

Troubleshooting Tips

Clarify for students the difference between a complete sentence and a fragment sentence. At this stage, it can be helpful for students to know when a fragment "works," as in writing about action, and when it does not, as in writing a more formal expository essay. The distinction can help students control their use of fragment sentences.

Plan on spending one or two 50-minute classes to introduce the concepts of motion capture, animation and action-writing followed by two or three 20-minute sessions to critique drafts of the written action sequence.

Activity Extensions

Learn how the Gollum character was "captured." Use the keywords "Gollum" and "motion capture" in Google and at the Google news desk. Report your findings to the class.

How about a career in computer animation using the latest in motion-capture technology? Chances are the field will have advanced considerably by the time you are ready for a career, but there is no time like the present to start learning about this exciting technology. For more information, see: https://kwhs.wharton.upenn.edu/2016/12/careers-motion-capture-industry-on-the-move/.  

Activity Scaling

  • Because this activity works through a succession of drafts, students of varying abilities can be rewarded for progress as well as achievement.


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More Curriculum Like This

Middle School Lesson
Rocking the Boat

The concepts of stability and equilibrium are introduced while students learn how these ideas are related to the concept of center of mass. They gain further understanding when they see, first-hand, how equilibrium is closely related to an object's center of mass.


The Advanced Computing Center for the Arts and Design (ACCAD). The College of the Arts, The Ohio State University. Accessed May 16, 2004. http://accad.osu.edu/

Center of Gravity, Science Snacks. Exploratorium, San Francisco, CA. Accessed May 16, 2004. http://www.exploratorium.edu/snacks/center_of_gravity.html

Chu, Chi-Wei (Wayne), Jenkins, O.C. and Matarić, M.J. "Markerless Kinematic Model and Motion Capture from Volume Sequences." Computer Vision and Pattern Recognition, Proceedings. 2003 IEEE Computer Society Conference. Volume 2, 8-20, pp. II-475-II-482, June 2003. (Source of motion capture center of gravity image)

Dictionary.com. Lexico Publishing Group, LLC. Accessed May 15, 2004. (Source of vocabulary definitions, with some adaptation) http://www.dictionary.com

Furniss, Maureen. Motion Capture. MIT Communications Forum. Accessed May 16, 2004. http://web.mit.edu/comm-forum/papers/furniss.html

Hulsey, S. B. "Kinko." Writing Action. Elfwood Tutorials: Fantasy Art Resource Project. Accessed May 16, 2004. (If the FARP site becomes inactive, contact the author via email: sbhulsey@mindspring.com) http://elfwood.lysator.liu.se/farp/thewriting/dra2action/dra2action.html

Li, William. Figure Drawing: Basic Pose and Construction. Elfwood Tutorials: Fantasy Art Resource Project. May 16, 2004. For questions, contact the author via email: williamli.art@outlook.com

Motion Capture Downloads. Pyros Pictures Mocap. Accessed May 16, 2004. (Start with "Head Tracking"; lots of video clips that show how it's done; excellent site but connection is sporadic) Formerly at: http://www.pyros.com/html/body_movies.html

Motion Capture Lab. The Advanced Computing Center for the Arts and Design (ACCAD). The College of the Arts, The Ohio State University. Accessed May 16, 2004. http://accad.osu.edu/research/mocap/mocap_home.htm


© 2004 by Regents of the University of Colorado.


Jane Evenson; Malinda Schaefer Zarske; Denise Carlson

Supporting Program

Integrated Teaching and Learning Program, College of Engineering, University of Colorado Boulder


The contents of this digital library curriculum were developed under a grant from the Fund for the Improvement of Postsecondary Education (FIPSE), U.S. Department of Education, and National Science Foundation GK-12 grant no 0338326. However, these contents do not necessarily represent the policies of the Department of Education or National Science Foundation, and you should not assume endorsement by the federal government.

Last modified: August 16, 2023

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