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Guide Index

Body Sense

The Magic Years?

Born to Talk

A Change of Mind

Speaking For Herself
in the classroom

IT'S A KID'S WORLD: Body Sense

How do babies learn to reach, kick, crawl and walk? In this story, we'll meet a psychologist exploring possible answers to that question by measuring infants who bounce up and down in a baby jumper. Other psychologists have long been fascinated by the same query, and have proposed various theories to explain why humans do what they do. Some rare archival footage from the 1930s looks at a classic study of twins, and a modern-day researcher computes muscle patterns.

Curriculum Links
Activity 1: Going Up?
Activity 2: Learning to Move



musculoskeletal system,
reflex arc

experimentation, trial and error

dexterity, gait,
gross motor movement

anatomy and physiology


chaos, repetition

coordination, kinesiology,
muscle development, training

recoil, springs

learning, memory


The recoil of infant bouncing systems depends upon the elasticity of support components. Some bouncing systems use metal coils; others are made of rubber-like materials. When these supports are stretched, the potential energy of the system is increased. When released, the potential energy transforms into the kinetic energy of the spring's contraction. This activity gives you a chance to investigate the elasticity of a metal coil toy, the ever-popular Slinky.

  • Slinky coil toy
  • meter stick
  • tape
  • stack of books
  • thin board
  • 50gm brass mass (or use modeling clay)

  1. Count off about 20 rings from the bottom of the coil. Open the coil at this point and insert a thin board. Be careful not to bend or damage the coil structure.

  2. Place the board on the edge of a table, so that the rings of the coil hang freely. Steady the overhang by placing a stack of books on top of the thin board.

  3. Secure a mass (approximately 50gm) to the bottom of the coil. The coil should stretch several centimeters.

  4. Tape the meter stick alongside the coil. Use tape to mark the bottom of the coil on the meter stick.

  5. Pull the coil down 3cm. Release and observe the highest point of its recoil. Record this value in a table that records:
    LENGTH OF STRETCH (from starting position)
    LENGTH OF RECOIL (from starting position)

  6. Repeat step 5 using these stretch distances: 6cm, 9cm, 12cm, 15cm.

  • What happens to the recoil as you increase the length of the coil stretch? (it increases)

  • What is the relationship between the stretch and recoil? (the spring recoils about the same distance as the stretch)

  • Why did you secure the 50gm mass to the coil? (you needed to stretch the coil to produce a compressible space for the recoil)


The ratio of a spring's stretch or compression to the amount of applied force is expressed mathematically by Hooke's Law: F equals kx. F equals the applied force; x is the amount of stretch or compression; and k equals the spring constant. After completing this activity, review Hooke's Law. Then, solve for the spring constant of the Slinky.

Compute the frequency per minute of your coil toy by counting the number of times it bounces in five seconds, then multiply by 12 (when a weight equal to that of a baby is put in the jumper, its "natural frequency" is calculated at 79 times per minute).


A baby doesn't just wake up one day and walk. That baby has spent months experimenting with movements and practicing different ways of moving its muscles. As you see on Frontiers, babies need to learn many things before they can walk; they especially need to practice coordinating their muscles. When you learn a new skill, you also need to practice and give yourself time to perfect the muscle patterns. Measure how practice helps you run faster with this activity.


  • stopwatch
  • measuring stick
  • cloth strip

  1. Divide your class into teams of four students each. Designate a time keeper for each team.

  2. Have each team measure and mark off a race course 20 meters long.

  3. Have each member run the course without any prior practice. Record the runner's time.

  4. Allow each team member to practice running for one minute and then run the course a second time. Record the new time.

  5. Have each team member repeat the course a third time. Record the time.

  6. Select two team members to run the course as a three-legged race. Secure the right leg of one student to the left leg of the other student. Have them run the course without practice and record the time.

  7. Have the other two team members practice running the course as a three-legged runner and then run the course. Record the time.

  8. Each of the three-legged groups should practice moving together for one minute, and then race again. Record the new times.

  9. All groups should practice moving together again for one minute and then race the course. Record the times.

  • How did the one-minute practice affect the running times of individual students?

  • How did one-minute practices affect the running times of three-legged teams?

  • Explain the role of practice in the two types of races.

  • What are some of the discrete skills the three-legged teams must learn?

  • What are some different ways a baby practices learning to walk?

  1. Calculate a runner's speed using the formula: speed equals distance/time. Use your team's data to determine each member's speed in meters per second, feet per second, kilometers per hour and miles per hour.
    (Note: your science textbook probably contains conversions between metric and English units. If not, consult another reference source.)

  2. Graph the collected data. The x-axis should display the trials. The y-axis should illustrate the speed of individuals and groups in miles per hour.

  3. Apply a similar sequence of practice to learning another skill, such as juggling, hopping on one foot, flipping POGs, writing your signature with your non-dominant hand or shooting baskets. What is the role of practice in learning? Can you correlate the amount of practice time with an improvement in the ability to perform the task?

  4. Imagine that a baby is a robot. Draw a flow chart that will instruct the baby on how to get from point A in the room to point B, or, from the table to the door. How many muscles will the baby have to coordinate to perform the task? Is it more efficient for the baby robot to crawl, scoot across the floor, walk by holding onto tables and other objects or walk unaided?


Scientific American Frontiers
Fall 1990 to Spring 2000
Sponsored by GTE Corporation,
now a part of Verizon Communications Inc.