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

By Mark Levi | Go to book overview

5
FLOWS AND JETS

5.1 Bernoulli's Law and Water Guns

Question. Imagine shooting water from a syringe by pushing the piston. With Newton's first law in mind (motion is steady if no force is applied), I ask: Does it take any force to move the piston with a constant speed—assuming a perfectly frictionless piston and perfectly nonviscous water? In other words, once I push the piston to give it some speed and let it go, will it continue at the same speed by inertia?

Answer. A force is required to push the piston with constant speed—even in a perfectly frictionless world. Newton's first law of steady motion by inertia doesn't apply here because some parts of fluid do accelerate—namely, those nearing the exit from the cylinder. Figure 5.2 shows this in more detail.

Figure 5.1. Will the piston move steadily by inertia if there is no viscos-
ity and no friction?

-39-

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

• 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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