Why Cats Land on Their Feet: And 76 Other Physical Paradoxes and Puzzles

By Mark Levi | Go to book overview

6
MOVING EXPERIENCES: BIKES, GYMNASTICS, ROCKETS

6.1 How Do Swings Work?

Question. Most things in life are easier said than done. But some are the other way around—easier done than said. Rocking on swings is an example. Exactly how does a child pass the energy of his muscles to swinging? The answer is not that obvious.1

Answer (the anatomy of a resonance). Imagine yourself rocking back and forth on swings. You feel the greatest g force when zooming past the bottom of your path. By the same token, the least g-force is near the highest points of your trajectory2 Imagine now holding a weight in your hand, resting it your lap, Figure 6.1. When you are passing the bottom of your trajectory, raise the weight to your shoulder; keep it there until you are near the top of the trajectory and once there quickly lower it back. Repeat: lift when passing

1 A little boy asked: [Grandpa, do you sleep with your beard over or under the blanket?] That night, the sleepless grandpa was lying on his back, trying his beard this way or that and cursing. Neither way felt right any more.

2 There is a [double whammy] responsible for the lesser g-force at the top of the trajectory: (1) the centrifugal force is smaller, and (2) gravitational force's component is smaller.

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Why Cats Land on Their Feet: And 76 Other Physical Paradoxes and Puzzles
Table of contents

Table of contents

  • Title Page iii
  • Contents vii
  • Acknowledgments xi
  • 1 - Fun with Physical Paradoxes, Puzzles, and Problems 1
  • 2 - Outer Space Paradoxes 5
  • 3 - Paradoxes with Spinning Water 17
  • 4 - Floating and Diving Paradoxes 28
  • 5 - Flows and Jets 39
  • 6 - Moving Experiences- Bikes, Gymnastics, Rockets 57
  • 7 - Paradoxes with the Coriolis Force 77
  • 8 - Centrifugal Paradoxes 84
  • 9 - Gyroscopic Paradoxes 104
  • 10 - Some Hot Stuff and Cool Things 117
  • 11 - Two Perpetual Motion Machines 127
  • 12 - Sailing and Gliding 132
  • 13 - The Flipping Cat and the Spinning Earth 142
  • 14 - Miscellaneous 146
  • Appendix 161
  • Bibliography 187
  • Index 189
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