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TEACHING GUIDES


THE WILD WEST: Looping the Loop


Rodeo sports such as bronco busting and calf roping are Western traditions derived from centuries-old occupations; knowing how to throw a lasso is essential both on the range and on the rodeo circuit. One rodeo competitor gets some valuable help from a hi-tech analysis of her technique and a practical experiment in physics. Using a computer and motion analysis, plus good old-fashioned practice, cowgirl Whisper Alexander learns how to throw a better lariat.

Curriculum Links
Activity 1: Testing Centripetal Force
For Further Thought




CURRICULUM LINKS

BIOLOGY

nervous system
GENERAL SCIENCE

human psychology
LIFE SCIENCE

skeletal muscles
PHYSICAL SCIENCE & PHYSICS

centripetal force
momentum and impulse
velocity
TECHNOLOGY

biomechanics
computers





ACTIVITY 1: TESTING CENTRIPETAL FORCE

As you saw on Frontiers, cowgirl Whisper Alexander applies physics to improve her roping skills. Sports physiologist Mike Meyers, also seen on Frontiers, helps Whisper and other competitors improve their rodeo performances.

In this segment, you observed the motion and physics of throwing a rope. When loops of rope are spun overhead, several forces interact. First, the arm generates a force to maintain the lasso's movement and shape of the loop. To keep the loop suspended in air, the upward force must at least equal the weight of the rope itself or the loop would fall. Centripetal force pulls the rope loop toward the center of rotation and maintains the loop's shape. In perfecting her loop, Whisper learns about these forces firsthand.

The following activity allows you to investigate and calculate centripetal force. In contrast to the rodeo loop, your gear will apply the force near the center of the spin. Yahoo!

Materials: hand
  • 1m kite string
  • duct tape
  • marker
  • scissors
  • washers
  • cork
  • 1/2 plastic straw (the plastic shell from a ballpoint pen also works)
Procedure:
  1. Attach one end of the kite string to a cork with duct tape. Thread the free end of the string through a section of straw.

  2. Thread the string through several small washers. Secure the washers with a knot.

  3. Pull the string so a 20cm section projects from the top of the straw to the cork. For reference, mark the string just above the top of the straw.

  4. Swing the cork in a small circle overhead. Record the number of revolutions the cork makes in a ten-second period.
Questions:

(Answers based on using a 20cm length of string.)
  1. What is the radius of the cork's circular motion?
    (20cm)

  2. What is the circumference of the circular path it travels along?
    (125.6cm)

  3. Using the data collected in step 4, calculate how many revolutions the cork completes in one second.

  4. In one second, how far does the cork travel?
    (multiply the answer to question 2 and the answer to question 3)

  5. What is the linear velocity of the cork
    (in meters per second)?

  6. What is the mass of the cork in kg?
    (answers will vary)

  7. Using the equation, Fc = m (v2/r), determine the centripetal force of the rotation.


Fc = centripetal force
r = radius
m = mass of cork
v = linear velocity




FOR FURTHER THOUGHT
  • If the string were to break, what direction would the cork travel? (In a straight line tangent to its last position.) Similarly, what would happen to you if you were riding in a car without a seat belt and the car took a curve too fast, causing the door to fly open and you to fly out?

  • Some physicists describe centrifugal force as a "fictitious force." Discuss the concept.
  • Where else would you be likely to experience centripetal force? (roller coaster, carnival ride)





 

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