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Terror in Space

Classroom Activity

To explore the concept of center of mass and experiment with how altering the location of an object's center of mass can affect its motion.

Materials for each team
  • copy of student handouts
    Controlling the Cube—HTML)
    NOVA Cube Template—HTML)

Part I

  • 1 Styrofoam ball
  • weights: pennies, washers or marbles
  • tape

Part II

  • scissors
  • tape
  • self-stick notes
  • 1.3-centimeter (0.5-inch) cube of modeling clay
  1. For Part I, organize students into pairs and distribute the student handouts and materials. Have students roll a Styrofoam ball across a table to a partner and observe the ball's path and motion. Encourage students to experiment with varying the force with which they roll the ball. Then have students add a weight to alter the location of the ball's center of mass, and have them roll and observe the ball again. Discuss how this activity relates to the behavior of the supply ship Progress as it attempted to redock with Mir.

  2. For Part II, distribute the NOVA Cube Template and clay. Have students cut out and assemble the cube. Students will experiment to determine the probability of a certain side of the cube landing face-up when the location of the center of mass is in the center of the cube, and again when it is off-center.

  3. For their main challenge, have students find how best to position the clay so that when they roll their cube, the NOVA logo appears face up the most often. Have them predict before experimenting. Once students have completed their experiments, have them present their results to the class and explain why they think their strategy worked.

Activity Answer

This lesson focuses on objects that are caused to rotate by some outside force. All objects in free fall (such as thrown balls and space ships in orbit) tend to rotate around their center of mass. When the center of mass doesn't coincide with the "center" of the object, the object's behavior becomes erratic, or "wobbly." Students should notice in Part I that the path and motion of the ball with added weight is erratic.

Unbalanced forces will cause the speed and/or direction of an object's motion to change. As students increase the force with which they roll the weighted ball, they will also notice that the ball's motion becomes increasingly erratic. This is essentially due to the fact that the frequency and amplitude of the "wobble" is greater if an object is moved with more force—thus making the motion seem more erratic. This activity models the behavior of the Progress as the cosmonauts attempted to redock it. Since the garbage was loaded off-center, the location of the supply ship's center of mass was no longer along its center axis. As the Progress was accelerated through space, the unbalanced center of mass possibly made the ship's motion increasingly erratic as it approached Mir.

In Part II, the theoretical probability that any one side of a cube will land face-up is 1 in 6 (assuming a cube has uniform sides and its center of mass is located at the "center" of the cube). When students add clay to the inside of the cube, the center of mass is no longer located in the center of the object. As the cube rolls, the side with the greater mass is more likely to be at the bottom, since the object is more stable in that orientation.

Links and Books


Sterling, Donna R., and Rebecca J. Graham. "And You Were There." Science and Children (March 1998): 41-46. Chronicles a sixth grade's simulated space mission. Students assumed the identity of a chosen mission's astronaut and followed the mission through to the landing and press conference. Extensive resource list.

Web Sites

NOVA Online—Terror in Space
Delves deeper into the program's content and themes, with features such as articles, time lines, interviews, interactive activities, resource links, program transcripts, and more.

NASA Shuttle/Mir
Provides information about the astronauts on board Mir and events occurring between March 1995 and June 1998. Includes sighting information.

Moscow, We Have a Problem
Transcript of a discussion from The NewsHour with Jim Lehrer (June 25, 1997) examining the future of joint missions between the United States and Russia in the wake of a crash between the Mir space station and a resupply ship. Includes an audio version of the segment.


The "Controlling the Cube" activity aligns with the following National Science Education Standards and Curriculum and Evaluation Standards for School Mathematics:

Grades 5-8

Physical Science

Science Standard B:
Physical Science

Motions and Forces:

  • The motion of an object can be described by its position, direction of motion and speed. That motion can be measured and represented on a graph.

  • An object that is not being subjected to a force will continue to move at a constant speed and in a straight line.

  • If more than one force acts on an object along a straight line, then the forces will reinforce or cancel one another, depending on their direction and magnitude. Unbalanced forces will cause changes in the speed or direction of an object's motion.

Math: Probability

Mathematics Standard 11:
Data Analysis and Probability

Grades 9-12

Physical Science

Science Standard B:
Physical Science

Motions and Forces:

Objects change their motion only when a net force is applied. Laws of motion are used to calculate precisely the effects of forces on the motion of objects. The magnitude of the change in motion can be calculated using the relationship F = ma, which is independent of the nature of the force. Whenever one object exerts force on another, a force equal in magnitude and opposite in direction is exerted on the first object.

Math: Probability

Mathematics Standard 11:
Data Analysis and Probability

Teacher's Guide
Terror in Space

Video is not required for this activity