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

By Mark Levi | Go to book overview

12
SAILING AND GLIDING

Question. Is it possible to sail on a river on a windless day?

Answer. Yes, sailing with no wind is possible, thanks to the current; Figure 12.1 explains how. The sail in still air acts like a knife in butter: it can only slice through the air, moving along the line of the sail.1 On the other hand, the keel is being pushed by the current, and thus the boat slides as shown, towards the shore at the right angle. So the keel now acts as the sail and the flowing water acts as the blowing wind. And the sail, slicing through the air, acts as if it were a keel! Its just like regular sailing, except upside-down.

The sail-keel symmetry. We just discussed the boat from the point of view of a shore observer. But imagine yourself on that boat; you will then think that the water is still and that the wind is blowing upstream instead. So you would be in a conventional situation of a boat sailing in the wind. To you the sail will act as a normal sail catching the wind, and the keel will be a normal keel slicing though water. This

1 This is an approximation—of course the sail can move in the direction perpendicular to the line S, but we ignore this smaller motion.

-132-

If you are trying to select text to create highlights or citations, remember that you must now click or tap on the first word, and then click or tap on the last word.
One moment ...
Default project is now your active project.
Project items

#### Items saved from this book

This book has been saved
Highlights (0)
Some of your highlights are legacy items.
Citations (0)
Some of your citations are legacy items.
Notes (0)
Bookmarks (0)

You have no saved items from this book

Project items include:
• Saved book/article
• Highlights
• Quotes/citations
• Notes
• Bookmarks
Notes

#### Cited page

Style
Citations are available only to our active members.

(Einhorn, 1992, p. 25)

(Einhorn 25)

1. Lois J. Einhorn, Abraham Lincoln, the Orator: Penetrating the Lincoln Legend (Westport, CT: Greenwood Press, 1992), 25, http://www.questia.com/read/27419298.

#### Cited page

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
Settings

#### Settings

Typeface
Text size Reset View mode
Search within

Look up

#### Look up a word

• Dictionary
• Thesaurus
Please submit a word or phrase above.

Why can't I print more than one page at a time?

Help
Full screen
/ 190

### How to highlight and cite specific passages

1. Click or tap the first word you want to select.
2. Click or tap the last word you want to select, and you’ll see everything in between get selected.
3. You’ll then get a menu of options like creating a highlight or a citation from that passage of text.

## Cited passage

Style
Citations are available only to our active members.

"Portraying himself as an honest, ordinary person helped Lincoln identify with his audiences." (Einhorn, 1992, p. 25).

"Portraying himself as an honest, ordinary person helped Lincoln identify with his audiences." (Einhorn 25)

"Portraying himself as an honest, ordinary person helped Lincoln identify with his audiences."1

1. Lois J. Einhorn, Abraham Lincoln, the Orator: Penetrating the Lincoln Legend (Westport, CT: Greenwood Press, 1992), 25, http://www.questia.com/read/27419298.

## Thanks for trying Questia!

Please continue trying out our research tools, but please note, full functionality is available only to our active members.

Your work will be lost once you leave this Web page.