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Flying circus of physics

Chap 1 (motion) archived stories part D

Monday, February 09, 2009

For Chapter 1, here is part D of the new stories and also the updates to the items in the book, including many video links and journal citations. If you want all the video links (hundreds) and journal citations (thousands) for this chapter, go to

First a list   (use "Ctrl-F" to search for a key word or just scroll down the screen)

1.161  Gee-haw whimmy diddle
1.162  Shot putting and the football throw-in
1.164  Pub trick ---removing a lighter from under a bottle
1.164  Pub trick --- removing a bill from between balanced bottles
1.164  Unusual ways to load and unload a truck
1.165  Pulling an airplane with teeth
1.168  Pub trick --- matchstick rocket
1.168  Riding a fire extinguisher through a subway car
1.172  Sliding a stick across fingers
1.185  Road corrugation
--------- New items (not in the book):
1.195  Falls over Niagara Falls
1.196  Backpacks on bungee cords are easier to carry
1.197  Projectile penguin from an ice hole
1.198  Swordplay
1.199  Car and train surfing

Reference and difficulty dots
Dots · through · · · indicate level of difficulty
Journal reference style: author, title, journal, volume, pages (date)
Book reference style: author, title, publisher, date, pages

Now the stories

1.161 Gee-haw whimmy diddle
Jearl Walker
Sep 2009 Here is a wooden toy that has probably amused children for at least a couple of centuries but which still perplexes physicists when they first see it. As you can see in my photo, it consists of a notched stick (about as wide as a pencil) with a propeller mounted loosely on a pin that is stuck into one end of the stick. I hold the notched stick in my left hand while with my right hand I run a second stick over the notches to cause vibrations.

Not surprisingly, the vibrations cause the propeller to vibrate and occasionally rotate one way or the other. However (and here is the surprise to both children and physicists), with a few magic words I can cause the propeller to rotate continuously in one direction or the other, or even to reverse directions at my command. An observer can look closely at what I am doing and yet, when given a chance with the toy, fail to make the propeller do anything but vibrate with an occasional brief rotation.

This rustic device has a variety of names, probably because it was popularized independently in many places. Some of the names in English are whammy doodle, wammy doodle, beano stick, gee-haw whimmy diddle, and whimmy diddle. Here are some videos that show how to construct the toy from either a pencil or a tree twig. pencil pencil twig same video, twig

If I rub the second stick over the top of the notches, the propeller merely vibrates up and down. To make it rotate continuously in one direction, I must make the extent of vibrations on the left and right sides of the stick different. To do this I rub the notches on either their left side or their right side instead of along the top, and I apply pressure to the notched stick with either my thumb or first finger.

To cause the propeller to rotate counterclockwise (in my view), I turn the notched stick slightly so that I rub along the right side of the notches while pressing my thumb against the right side of the stick. That side of the stick is then unable to vibrate as freely as the left side, and this lack of left-right symmetry causes the pin to rotate counterclockwise in an elliptical path. The friction between the pin and the hole in the propeller then causes the propeller to rotate in that same counterclockwise direction.

To reverse the rotation, I subtly turn the notched stick slightly so that I then rub the left side of the notches. At the same time I press against the left side of the stick with my first finger (rather than with my thumb on the right side). Now the left side cannot freely vibrate and the pin moves in a clockwise elliptical path, driving the propeller in a clockwise rotation.

Shifting from one side of the notches to the other and shifting between thumb pressure and finger pressure are so subtle that most onlookers do not notice the shifts even if they view the toy closely. So, you can make the most outlandish arguments for why you can cause the propeller to rotate and to reverse its rotation at your command. demonstration description purchase news item purchase

Dots · through ··· indicate level of difficulty
Journal reference style: author, title, journal, volume, pages (date)
Book reference style: author, title, publisher, date, pages
· Leonard, R. W., "An interesting demonstration of the combination of two linear harmonic vibrations to produce a single elliptical vibration," American Journal of Physics, 5, 175-176 (1937)
· Miller, J. S., "The notched stick," American Journal of Physics, 23, 176 (1955)
· Laird, E. R., "A notched stick," American Journal of Physics, 23, 472-473 (1955)
· Scott, G. D., "Control of the rotor on the notched stick," American Journal of Physics, 24, 464-465 (1956)
· Welch, S., "What makes it turn?" Physics Teacher, 11, 303 (1973)
·· Burger, W., "Elementary dynamics of simple mechanical toys," Mitteilungen der Gesellschaft fur Angewandte Mathematik und Mechanick, 2, 21-60 (1980)
· Mulder, D. P. J., "Children's toys," Physics Teacher, 18, 134-135 (1980)
· Aubrecht II, G. J., "A mechanical toy: the gee-haw whammy-diddle," Physics Teacher, 20, 614-615 (1982)

1.162  Shot putting and the football throw-in
Jearl Walker
July 2006
    As discussed in the book, the optimum angle for putting a shot in shot put (I love to say that) is not the intuitive 45 degrees for two reasons: (1) The shot is launched at a point above where it lands. (2) The shot putter can accelerate the shot to a greater launch speed at a shallower angle just because the orientation of the arm is less awkward.
     The same argument governs the throw-in of the ball in football (known as soccer in the U.S.). To throw the ball in from the sideline to teammates deep in the field, a player uses an overhead throw at a relatively shallow angle, perhaps 30 degrees. This launch angle is large enough for the ball to clear the heads of nearby opponents and is small enough that the player can greatly accelerate the ball during the launch.  

· ·  Linthorne, N., “A new angle on throwing,” Physics World, 19, No. 6, 29-30 (June 2006)
· · ·  Linthorne, N. P., and D. J. Everett, “Release angle for attaining maximum distance in the soccer throw-in,” Sports Biomechanics, 5, No. 2, 243-260 (July 2006)

Want more references? Use the link at the top of this page.

1.164  Pub trick --- removing a lighter from under a bottle
Jearl Walker
Dec 2007 
   Here is a cute bar trick (pub trick) from

An empty beer bottle is balanced upside down on a common cigarette lighter. (You can, of course, use a pop bottle balanced on some other narrow object with a fairly smooth surface.) How can you remove the lighter without toppling the bottle and without touching the bottle?

As the video shows, the technique is to quickly slide your index fingers toward you along the bar top so that they hit the opposite ends of the lighter simultaneously, knocking it toward you. The bottle then simply drops straight down. It may wobble slightly but it quickly stabilizes.

Now, anyone can do this trick but the power of physics is that you can also explain it. Here is the physics. If you were to gradually pull the lighter toward you with your index fingers, the friction between the lighter and the bottle would cause the bottom part of the bottle to move toward you also. Then the bottle would lean and, of course, it would then be unstable and would topple over.

The coefficient of friction between the lighter and the bottle is fairly high for a static situation, that is, for a situation where the two surfaces do not slide past one another. That is why your gradual pull on the lighter causes the bottom of the bottle to move. The coefficient of friction is less for a sliding situation. So, to avoid moving the bottom of the bottle toward you, you need to cause sliding immediately. Thus, you yank the lighter, making it slide, and the amount of friction is then too low to move the bottom of the bottle. So, the bottle just drops.

1.164  Pub trick --- removing a bill from between balanced bottles
Jearl Walker
June 2008
     Here is another video in the FCP series of pub trick explanations. A currency bill lies between an upright bottle and an inverted bottle balanced on top of it. The challenge is to remove the bill without upsetting the balance of the bottles. the original video the same video but posted on YouTube

The video shows that you hold firmly onto one of end of the bill and then with the other hand, strike down sharply with one finger onto the length of bill between the bottles and the held end. The bill rapidly slips out from between the bottles without the top bottle being knocked over. If, instead, you pull on one end of the bill, even sharply, the top bottle topples over.

One message of the Flying Circus of Physics is that anyone can do a trick --- the real challenge is to explain the trick. Here you might recognize that the physics is the same that lies behind the commonly seen tablecloth trick in which a tablecloth is pulled from beneath a set of dishes without the dishes being pulled onto the floor. In either situation, the secret is to set up kinetic friction instead of static friction.

In the bar trick, there is some adhesion between the bill and the lip of either bottle, and neither the bill nor the lips are perfectly smooth. So, if you pull the bill to one side, that adhesion and the slightly rough surfaces mean that the bill will grab the lip on the top bottle and move it in the bill’s direction of travel. Of course, that tilts the top bottle, eliminating its balance.

If, instead, you suddenly move the bill very quickly, the adhesion is almost immediately broken and the top bottle is lifted very slightly by the first set of ridges. Then ridge after ridge slip between the top bottle without any of them butting against the lip, until the bill is entirely free of the bottles.

We distinguish friction between two surfaces by a coefficient of friction, a number lying between 0 (for no friction) and, say, 1.2 (for the high friction between rock and rock-climbing shoes). When we attempt to move one surface over the other but there is no motion, we say that the “grab” between them is due to the many points of adhesion and is a static friction, with a coefficient of static friction. We often do not know why points are adhering but the reason is usually due to an electrical interaction between the atoms of one surface and the adjacent atoms on the other surface.

If we pull hard enough so that the surfaces slide across one another, the adhesion is reduced but not eliminated. There are now points with fleeting adhesion, bonds that are formed and almost immediately broken as the surfaces slide past each other. When the bill is hit smartly by a finger in the bar trick, the bonds in the contact regions between the bill and either bottle are suddenly broken and only the fleeting bonds work against the bill’s motion. Those fleeing bonds are too few (and thus the friction is too weak) to move the ends of the bottles by more than a millimeter. Thus, the bottles are never tilted enough to fall over.

· Hudson, H. T., "There's more to it than inertia," Physics Teacher, 23, 163 (1985)
· Haber-Schaim, U., and J. H. Dodge, "There's more to it than friction," Physics Teacher, 29, 56-57 (1991)

1.164  Unusual ways to load and unload a truck
Jearl Walker
April 2013  There are, of course, many ways to unload a truck, from unloading by hand, piece by piece, to tilting the bed of the truck so that granular material such as gravel can flow out. But here is a way that resembles the common physics demonstration in which a table cloth is whipped out from underneath dishes without spilling the dishes. The idea is to yank the cloth so as to set up kinetic friction between the cloth and the plates. In that case, the frictional forces will be less than the upper limit to the static frictional force. With smaller frictional forces acting on them, the dishes will not move very far while the cloth is pulling on them and thus will not be pulled off the edge of the table.

This truck driver uses this physics to remove the load of long bamboo rods from his truck. He quickly backs up the truck and then slams on the brakes. The rearward motion of the load overwhelms the static frictional forces between the load and the truck bed, allowing the load to slide over the bed.

When the driver repeats the process, the load slides far enough to the rear that it tilts downward and onto the ground. Then the driver accelerates forward in spurts, again setting up kinetic friction and allowing the bed to slide under the load. Eventually, the truck pulls out completely from under the load.

Here is another unusual technique. The rear edge on a long, covered truck is lowered to the ground to act as a scoop as the truck backs toward a long cylinder of bundled material. The cylinder consists of several sections. Inside the truck, a treadmill helps move the cylinder forward.

After the full cylinder is inside the truck, the treadmill is reversed so that the cylinder moves rearward as the truck moves forward. Now the rear edge is elevated. As each cylindrical section passes over the edge, it is torqued by the gravitational force so that it lands upright on the ground.

The truck has converted all the individual cylinders from being horizontal to being vertical. Finally the truck backs up with the rear edge low to the ground so that the upright cylinders are loaded. What I want to know is, who thought of this?

1.165  Pulling an airplane with teeth
Jearl Walker
August 2007  The first of the following links takes you to a video shot in Bhopal, India, where Rishi Saxena is shown pulling an airplane by clamping a rope in his mouth, facing away from the airplane, and leaning against the rope while also pushing against the tarmac with his legs. Thus, he uses both his weight and his leg muscles to put tension in the rope. The rope is almost horizontal, which makes the effort efficient. Had the rope been angled upward, part of the force he applies would have been upward, which is useless. Had the rope been angled downward, part of the force would have been downward, just as useless.

The hard part is to get the airplane moving without the feet slipping on the tarmac. Once the plane is moving, Saxena can afford to move his feet forward and effectively fall onto the rope again and again while continuing to push against the tarmac. 

I have included more links to videos of people pulling airplanes, which perhaps is the thing to do when there is just nothing but those "reality shows" to watch on television. If you want to see even more pulling images, including images of John Massis of Belgium, who was legendary for pulling a locomotive with his teeth, check the other links here. The record pull by Massis is described in The Flying Circus of Physics. Rishi Saxena  A team pull a heavy airplane Pulling a FedEx airplane Photo Photo airplane pulled by a team

1.168 Pub trick --- matchstick rocket
Jearl Walker
Oct 2009   Well, maybe this is less of a pub trick and more of a sure-fire way to get yourself thrown out of a pub. Indeed, if you make one of these rockets, you need to be very careful that you do not start a fire or hit someone in the face (or much worse, in the eyes).

As you can see in this video, a matchstick rocket is made by enclosing a match head with a small strip of aluminum foil and then holding a burning match under the foil. The match head finally ignites, the matchstick suddenly and dramatically shoots away, traveling as much as 10 m. (Thus, unless care is taken, it can injure a bystander who is unaware of the danger.)

Normally a match is ignited by rubbing it sharply along a striking surface attached to the matchbox or matchbook. The striking surface contains phosphorus, which is ignited by the heat generated by the friction in the rubbing. The match head contains potassium chlorate, which releases oxygen when heated by the phosphorus and the rubbing. The match head also contains a small amount of sulfur, which is ignited by the heat. Although the match head is normally surrounded by air, the release of oxygen by the match head itself guarantees that the sulfur will burn, which then ignites the wood (or paper) of the match.

The burning procedure is a bit different in a matchstick rocket. First, there is no rubbing or burning of phosphorus to cause an oxygen release by the potassium chlorate. Instead, the heat from the already burning match below the match head causes that release. Second, because the match head is enclosed with aluminum foil, the burning depends entirely on the oxygen release.

That release and the burning of the sulfur (and perhaps other components in the match head) produces a dramatic release of gas, which escapes along the sides of the match stick, through either chance openings in the aluminum-foil covering or one that is purposely left there.

To make sense of the propulsion of the rocket, we consider its momentum, which is the product of mass and velocity. Initially, because the rocket is stationary, the momentum is zero. The only way that the momentum can be changed is if an external (outside) force acts on the matchstick. Here, there is no such external force. There are chemical reactions with a production of gas, but those are all internal to the rocket. Without any external force to change the momentum of a system, we say that the momentum of the system must be conserved (that is, stay the same).

However, when the gas is suddenly produced and escapes down along the matchstick, it has momentum toward the rear of the rocket. Because the system of the stick and gas must continue to have a total momentum of zero (the initial value), the stick itself must move in the forward direction with just as much momentum as the gas has in the rearward direction. If the aluminum did not force the gas to escape toward the rear, the propulsion of the matchstick would be eliminated. Heating the match head to its ignition point would then simply ignite the match --- no fun there.

Momentum may not be a warm and fuzzy concept to you because it does not seem to be as apparent as, say, a force on your arm. And yet, the conservation of momentum is the underlying reason by the matchstick shoots out in such a mischievous way, delighting countless young boys ever since matches were invented almost 200 years ago. a powerful match rocket making of a two-match rocket, video 1 flight of the two-match rocket, video 2 three-match rocket, video 3


1.168 Riding a fire extinguisher through a subway car
Jearl Walker
Dec 2010  Here is a viral video that has generated a tremendous number of hits, but is it real? Three rowdy young people board a subway, and one of the young men unhooks a fire extinguisher from a wall, mounts it as though he is on a horse, and then pulls its lever. As the extinguisher shoots out its fire-extinguishing foam, it apparently shoots forward like a rocket, down the aisle of the subway car, with the young man along for a ride and passengers scurrying out of the way. main video shorter video, different perspective


You can explain the apparent motion either of two related ways:

(1) The chemical reactions within the extinguisher produce a force that propels the foam rearward and out the nozzle. A force with the same magnitude pushes against inside of the cylinder, propelling it forward. (This is an example of the third law of motion by Isaac Newton.)

(2) The chemical reactions propel the foam outward with a certain amount of momentum. They also give an equal amount of momentum to the cylinder in the opposite direction. (This is an example of the conservation of momentum, which is related to Newton’s third law.)

A more familiar example of this physics occurs every time a firearm is fired. The chemical reactions in the explosion within a cartridge shell propel the bullet in one direction while they also propel the firearm in the opposite direction, a motion commonly called recoil. With a firearm that gives a significant amount of momentum to the bullet, the recoil can be surprisingly large and even harmful. Here are some examples, where recoil comes as a surprise. Powerful rifle. I don’t speak the language and thus don’t know if the audio contains any rude words. girl firing rifle

Here are some more examples, including larger weapons. artillery recoil old cannon

Using a fire extinguisher as a small-scale rocket has long been the staple of a physics classroom, especially on the days of a public show. The extinguisher is fastened to a lightweight cart on wheels. The instructor sits on the cart and then opens the lever on the extinguisher. The instructor and cart are then propelled across the room. Here is one example. fire extinguisher, trolley through lecture hall

Now, back to the original prank of rocketing along a subway aisle. Any thoughts? Anyone can call out “fake,” and indeed such callouts were frequent when the video of the prank was posted on Youtube. But where’s the proof? How about this?

(1) The ejection through the nozzle is not vigorous. In the classroom demonstration, the instructor who rides the cart is in no danger of being slammed into the far wall at high speed, like the coyote in a Roadrunner cartoon. (Alas, perhaps such a dramatic crash is anticipated by the class, especially after a difficult exam, but it never happens.) Besides, if the emission were vigorous, using a fire extinguisher to put out a fire would be very difficult because it would fly out of your hands every time you turned it on.
(2) To help support the young man on the subway, the nozzle should be directed downward. Instead, it is tilted somewhat upward.
(3) When the young man falls off the cylinder, the cylinder drops to the floor where it lies like a sleeping dog. If its emission were vigorous, it would thrash about, running into the seats and bouncing off into new directions.

So, the rowdy young people made a fake video. Indeed, we can guess that they made the video while skipping their physics class.

Could you possibly use, say, two extinguishers pointing straight down to lift yourself off the ground? And if not with two, how many 10? No, no, wait. How about 100? Or even 200? Well, here is a link where that is actually tried.

 More links short video, big caliber rifle shotgun woman shotgun boy big bore large bore knocked off feet handgun into head fire extinguisher and wagon sled (going downhill helps too, don’t you think?) fire extinguisher in train fly through train part 2


1.172  Sliding a stick across outstretched fingers
Jearl Walker
January 2015  Hold a meter stick horizontally on your index fingers, with the fingers at opposite ends of the stick, and then move your fingers uniformly toward each other. Does the stick slide uniformly? No, it alternates between sliding on one finger and then the other, changing several times before the fingers reach the center of the stick. Why?

Here is a video with a yard stick:

And here is a video with a push broom:

In spite of appearances, the initial conditions on the fingers are not symmetric. You inevitably pull slightly harder with one finger—say, the right one—and overcome the static friction on it from the stick, and so it begins to slide beneath the stick. The friction on it is then kinetic friction, which is initially smaller than the static friction on the left finger. But as the right finger moves toward the center, the portion of the stick’s weight that it supports increases, and so does the sliding friction, until the friction there exceeds the friction on the left finger. Then the right finger stops and the left finger begins to slide. Soon the left finger supports so much weight that it stops and the right finger begins to move again. The cycle is repeated until your fingers get near the stick’s center, and then the stick tends to topple off your fingers. fingers sliding under meter stick

Dots · through ··· indicate level of difficulty
Journal reference style: author, title, journal, volume, pages (date)
Book reference style: author, title, publisher, date, pages
· Sommerfeld, A., Mechanics, Academic Press, 1964, pages 83-84
·· Mancuso, R. V., "Quantitative analysis of moving two fingers under a meterstick," Physics Teacher, 31, 222-223 (1993)
·· Massalha, T., Y. Lanir, and P. Gluck, "Moving fingers under a stick: a laboratory activity," Physics Education, 46, No. 2, 211-214 (March 2011)


1.185  Road corrugation

Jearl Walker
April 2008
   Here is something that drives road engineers nuts but utterly fascinates many physicists. Oh, but be warned that this sounds as though it is terribly boring. In fact, to normal people, it is terribly boring.

No matter how carefully a dirt or gravel road is smoothed, as cars and trucks travel along it, it develops a series of ridges across its width, said to be road corrugation. If you live in the country you already know that driving along a corrugated dirt road is much like riding a bucking wild horse in a rodeo. If you are not wearing a seatbelt, you can be slammed against the ceiling of the vehicle. photo of corrugated dirt road

Whenever a pattern appears unexpectedly or mysteriously, the pulse of a physicist quickens. Indeed, I often think that physics is largely a hunt for patterns in the world, somewhat like a heavy-metal fan hunts for the ultimate guitar rift.

Recently Nicolas Taberlet (University of Cambridge and Laboratoire de Physique in Lyon), Stephen W. Morris (University of Toronto), and Jim N. McElwaine (University of Cambridge) devised a neat experiment about road corrugation. Rather than deal with the complexity of a real car, they arranged for a circular pan of sand to rotate under a wheel made of hard rubber. As the moving sand rubbed the wheel, the wheel rotated. The arrangement, photos, and videos are available at

The researchers could control the speed between the sand and the wheel and could vary the type of sand and the diameter of the wheel. What they found surprised me because every since I began The Flying Circus of Physics decades ago, I thought that road corrugation was due primarily to the suspension system (the springs) of the car. However, in the simplified arrangement of these researchers, there is no spring.

When the speed was below a critical value of about 1.5 m/s, the initially flat sand remained flat. When the speed was increased to somewhat above the critical value, ripples (ridges) began to appear in the sand and spread around the circular path left by the wheel as the pan rotated below it. As they spread, the ripples began to overlap and then they settle into a permanent pattern. At this speed, the wheel continuously makes contact with the sand. At even greater speeds, the ripples still appeared but now the wheel became airborne as it shot over a ripple.

I can understand why a pattern develops if the wheel becomes airborne, because when it lands the wheel digs into the sand and then must climb the mound in front of it, where it becomes airborne again. What I cannot understand is the ability of the wheel to set up a pattern without becoming airborne. I can only guess that some initial nonuniformity in the sand, say, some imperceptible ridge, gets repeatedly hit by the wheel and pushed up to where it can be seen. And then, as the wheel travels over and down the now larger ridge, it tends to gradually dig out a valley and then another ridge.

The researchers found that the pattern formation was independent of the texture of the sand (indeed, in some of their experiments they used rice) and the wheel diameter. If you are an amateur scientist, here is an experiment you might consider. Maybe you can find a way to explain why the patterns begin. Video of a lab demonstration showing road corrugation Article by Ned Rozell Article by Ken Skorseth

· Schilling, R., and H. O. Fuchs, "Modern passenger-car ride characteristics," Journal of Applied Mechanics ASME, 8, A-59 — A-66 (1941)
· · Cox, H. L., "A peculiar function arising from an everyday problem," Mathematical Gazette, 35, 48-49 (1951)
· Mather, K. B., "Why do roads corrugate?" Scientific American, 208, 128-136 + 172 (January 1963)
· · · Kurtze, D. A., “The genesis of washboard roads,” International Journal of Modern Physics B, 15, Nos. 24 & 25, 3344-3346 (2001)
· · · Both, J. A., D. C. Hong, and D. A. Kurtze, “Corrugation of roads,” Physica A, 301, 545-559 (2001)
· · · Egger, J., “Hump pattern generation on ski slopes,” Physica D, 165, 127-136 (2002)
· Shoop, S., R. Haehnel, V. Janoo, D. Harjes, and R. Liston, “Seasonal deterioration of unsurfaced roads,” Journal of Geotechnical and Geoenvironmental Engineering, 132, No. 7, 852-860 (July 2006)
· Rehmeyer, J., “Road bumps,” Science News, 172, 102 (18 August 2007)
· · Taberlet, N., S. W. Morris, and J. N. McElwaine, “Washboard road: the dynamics of granular ripples formed by rolling wheels,” Physical Review Letters, 99, article # 068003 (4 pages) (10 August 2007) Results, photos, video available at

1.195  Falls over Niagara Falls
Jearl Walker
August 2006   Many people have tried to ride contraptions (balls, tubes, and other shapes) over the edge of the Canadian side of Niagara Falls. Most paid with their lives; the others, especially in recent times, paid heavy fines for their stunts. What is deadly about the fall?
    The fall itself, which lasts about 3 seconds, can be jarring because the contraption hits hard enough that the water cannot easily move out of the way. Still, the impact could be survivable, especially if a rider is surrounded by padding, which would prolong the collision and thus decrease the size of the collision force on the rider. (Because the force is inversely proportional to the duration of the collision, prolonging that duration by the use of padding decreases the size of the force.) 
    However, if the contraption hits the rocks at the bottom of the falls, the impact is so abrupt that survival is unlikely, even with considerable padding. The contraption might even bounce, which could throw the rider around the contraption or severely jar the rider, compounding the danger. In the past, the stunt people had to contend with an additional, subtle danger. In those days the water flow at the bottom of the falls could submerge a contraption behind the falling water. Several people drowned after their contraption became trapped like this and then filled with water. John Lienhard essay

· Warwick, P. D. A., “Over the falls in a barrel,” Invention & Technology, 10, No. 4, 34-43 (Spring 1995)

1.196  Backpacks on bungee cords are easier to carry
Jearl Walker
February 2007
   The main reason that a heavy backpack is difficult to carry is that as you walk, your torso rises and falls by several centimeters and thus you must repeatedly accelerate the backpack through that vertical distance. The force on your back is greatest when the torso begins to move upward and the backpack is accelerated upward. That demand automatically limits the speed at which you can walk, and running, such as running after a bus, is usually out of the question. More serious, the demand can severely limit an emergency crew member from moving rapidly with a heavy backpack of rescue equipment.
       However, the difficulty of moving with a heavy backpack can be significantly reduced if the backpack is suspended by bungee cords from a pack frame. A person with such arrangement can even run. Although you might think a suspended backpack would be unwieldy, research conducted by Lawrence C. Rome, Louis Flynn, and Taeseung D. Yoo of the University of Pennsylvania showed that the vertical motion of the load is significantly decreased when the load is suspended than when it is rigidly attached to the back in the normal fashion. The reduction is primarily due to the out-of-step oscillations of the load with the torso. That is, when the torso moves upward, the load moves downward, and vice versa. There are two results: (1) The reduced vertical oscillation means that less force is required to accelerate the load. (2) When the load begins to move upward, the bungee cords accelerate the load instead of only the torso accelerating it.

This physics is similar to that discovered long ago in Asia, where some people carry light to moderately heavy loads by tying them to opposite ends of a spring pole such as a bamboo pole and placing the center of the pole over one shoulder (check the URL listed below). As I discuss in the Flying Circus book, when such a person walks or runs, the two loads oscillate vertically and out of step with the vertical oscillation of the supporting shoulder. As a result, less force is required of the shoulder in the upward portion of the motion, because the springy pole helps propel the loads upward.
URLs  walking with backpack  running with backpack  Photo, bamboo pole use

· Rome, L. C., L. Flynn, and T. D. Yoo, “Rubber bands reduce the cost of carrying loads,” Nature, 444, No. 7122, 1023-1024 (21/28 December 2006)
· Harder, B., “A backpack with a suspension system,” Science News, 171, No. 2, 29 (13 January 2007)

1.197  Projectile penguin from an ice hole
Jearl Walker
February 2007    For those of you using Fundamentals of Physics, the textbook that I write, you know that I am greatly amused by the deadpan comic look of emperor penguins. But those penguins must be very smart to survive in their extremely harsh environment. Here is one example. When a penguin returns from foraging for food in the water, it might need to leave the water through a hole in an overlying ice layer. If the ice surface is only a centimeter or so above the water surface, the penguin just plops out onto the ice and then wiggles its way free of the water. However, if the ice surface is higher, the penguin must “leap” from the water, much like you might leap across a stream.

      To leap, the penguin moves toward the hole either almost vertically (if the hole is narrow) or at an appreciable angle to the vertical (if the hole is wider). Buoyancy alone may propel it sufficiently, but if the ice surface is high, the penguin must increase its speed by stroking. In a successful leap, the penguin clears the hole in a parabolic path and lands belly-first on the ice, as you can see with this link. Gosh, life might be tough in a physics class, but at least you don’t have to shoot yourself through a hole in an ice sheet to survive. (Well, you won’t have to IF you graduate.)  

      Students using my textbook or any other introductory physics textbook learn how to calculate the launch speed and launch angle that is required for such projectile motion if an object must reach a certain height, here the height of the ice surface. They also learn that during the flight, an object’s initial kinetic energy is converted to potential energy as the object rises. Students usually struggle with both ideas and are quick to slide open their calculators for the “equation-solving” function available on one of the keys.

       An emperor penguin can make the calculation without any equation-solving calculator. (Just imagine a penguin trying to push in those little keys.) Instead, a penguin can judge the height of the ice above the water and then mentally determine what speed is required to reach the top of the ice and thus whether it should just be buoyed upward or if it should stroke for a greater speed. The leap can be critical because the penguin is in a hostile environment and must be able to escape from the water on the first try and with as little wasteful expenditure of energy as possible. If it fails to leap up onto the ice, it falls back into the water, where it might become the lunch of a predatory seal. (Becoming something’s meal is a great motivator to do calculations correctly.)

      Depending on the angle of launch, there is always a least speed to reach a given height of ice. However, a penguin usually uses a somewhat greater speed because its view of the height of the ice can be misleading. When the light from the top of the ice travels down through the water surface, its path is bent (refracted) toward the vertical. When a penguin looks back along the light to make sense of its origin, the ice looks higher than it truly is. So, the penguin uses a speed that is greater than it truly needs. This overshoot costs the penguin some extra energy, which is precious in that harshly cold environment, but the cost is minor compared to the cost of being eaten by a seal.

· · Yoda, K., and Y. Ropert-Coudert, “Decision-rules for leaping Adelie penguins (Pygoscelis adeliae),” Journal of Zoology, London, 263, 1-5 (2004)
Sato, K., P. J. Ponganis, Y. Habara, and Y. Naito, “Emperor penguins adjust swim speed according to the above-water height of ice holes through which they exit,” Journal of Experimental Biology, 208, 2549-2554 (2005)


1.198  Swordplay
Jearl Walker
April 2007     Physics is everywhere (surely that is already a tee-shirt statement), and here is a quick example that must have been wrung from the deaths of many men. Originally swords were just long daggers with flat blades. Then a blacksmith somewhere discovered the physics of repositioning some of the mass of a sword. The blacksmith ran fullers (grooves or depressed regions) along the length of the blade on one or both sides. However, the intent was not to lighten the sword because the mass was simply shifted to form the pommel, the ball at the near end of the handle as you would hold it. You can see the pommel on the sword "Anduril" held by Aragorn  in this image from The Lord of the Rings.
    The addition of a fuller reduces the cross-sectional area of the blade, reducing the blade’s strength (the ability to withstand a sharp impact without breaking). However, transferring the metal to the pommel more than makes up for that reduction because the mass is then much closer to the wrist around which the sword must be rotated in a fight. When mass is moved closer to a rotational axis, the rotational inertia (or moment of inertia) is decreased. That quantity is a measure of how difficult rotation is. If you attempt to rotate, say, a long stick with a block taped to the outer end, the rotation is difficult. If you move the block closer to your hand, reducing the rotational inertia, the rotation is much easier. Thus, with the mass moved from the blade to the pommel, the sword is much easier to rotate rapidly in a fight.
    The pommel also helps reduce the jolt or sting felt by the hand holding the sword when the sword collides with a target. This same type of jolting is felt in any sports game where a ball is hit with a bat, stick, or racket. Suppose that you hold your sword horizontally as your opponent’s sword slashes down on it. The impact tends to move your sword downward and also rotate the handle upward around a point somewhere out on the blade. Which way the handle moves depends on how far out on the blade the collision occurs. If it is near your hand, the downward displacement dominates and your hand is jolted downward. If the impact is out near the sword tip, the upward rotation dominates and your hand is jolted upward.
    Somewhere along the blade there is a sweet spot or (center of percussion) where an impact does not jolt your hand at all because the downward displacement and the upward rotation cancel and the handle does not move. Of course, in a fight, you cannot adjust where the impacts occur along your blade and so you certainly must suffer the jolts without losing control of your sword. However, you can reduce the severity of the jolts by using a sword with a pommel. The pommel’s extra mass on the handle decreases both upward rotation and downward displacement, regardless of which dominates. Thus, with a pommel on the sword, you are less likely to have the sword knock out of your hand. Winning a sword fight when your sword is on the ground is just really difficult. 
    Physics is everywhere, even in a sword fight. There, that would make a most excellent tee-shirt statement.

· Denny, M., “Swordplay: an exercise in rotational dynamics,” European Journal of Physics, 27, 943-950 (2006)

1.199  Car and train surfing
Jearl Walker
May 2007
   Here is something so stupid that I will not even make a joke about it. Some young men stand on the hood or top of a moving car, the back of a moving pickup truck, or the top of a moving train (as in this photo by Petazeta). And, of course, most of them end up dead or seriously injured, usually brain injured. Now, presumably they were trying to show off. Well, being dead or brain injured certainly shows off something, but it is not intelligence or even masculinity. I’m really hoping that your common sense will protect you from something this idiotic, but just in case you need a scientific argument, let’s take a quick look at the physics of standing.

When standing, you are stable if your center of mass (the center of the distribution of mass in your body) is over an imaginary oval that spans the floor between your feet and extends a few centimeters forward and rearward. If someone tries to push you over, the person must push your center of mass until it is no longer above this stability oval, as we can call it. The person has a much easier job if they push you along the narrow axis of the oval, because then the center of mass needs to be moved only a few centimeters to make you unstable. The job is more difficult if they push you along the long axis of the oval, because then the center of mass needs to move tens of centimeters. Any good martial-arts fighter knows this lesson. The fighter partially squats with feet well separated and then guards against a push along the narrow axis of the stability oval, where an attack will probably come.

If you have ever ridden a lurching bus while having to stand, you have instinctively solved your stability problem by moving one foot forward and the other rearward. That way the long axis of your stability oval is parallel to the bus’s direction of travel, and you can fairly easily react to any sudden acceleration or braking without falling over. However, you have almost no stability against any sideways lurch of the bus, which could easily cause you to fall if you are not holding onto a pole or strap. This is something you definitely do not want to do if you will fall onto a burly guy with eyes glazed over from a lifetime of chemical abuse.

Now think about the stability of someone “surfing” on a moving car, truck, or train. The surfer instinctively stands as you do on a lurching bus and he feels like he has great stability against any forward or backward motion of the vehicle. However, he apparently does not realize that he has almost no stability against a sudden sideways lurch, as can happen when the vehicle hits a bump or some other imperfection in the pathway.

I promised not to joke about this stunt, so I won’t even mention the Darwin Award, for which this would be a surefire winner. I’ll just end with this challenge: If you want something that is really dangerous and life threatening, something that will shake your internal resolve and curl your hair, something that will scare you until you almost lose control of your bladder, then take a PChem course!

Come on. I dare you! No, I double-dog dare you!! train surfers of Soweto. They dance and laugh, all in great fun, but if the train had lurched due to sudden braking or bad railing, the center of mass of each surfer would have moved past the support area and the person would have fallen off the train. This is the most famous train-surfer video, but “trainrider” uses suction cups to maintain stability on the high-speed train. Don’t believe the cancer story. Here is the inside story of “trainrider”--- no cancer, just a deep disregard for safety. News story about a subway surfer that was killed.

· Peterson, T., G. Timberlake, A. Yeager, M. Jadali, and K. Royer, “Car surfing: an uncommon cause of traumatic injury,” Annals of Emergency Medicine, 33, No. 2, 192-194 (February 1999)
· Allan, R. S., P. J. Spittaler, and J. G. Christie, “’Ute surfing’: a novel cause of severe head injury,” Medical Journal of Australia, 171, Nos. 11-12, 681-682 (6 December 1999)
· Sternick, I., R. D. Gomes, M. C. Serra, H. N. Radwanski, and I. Pitanguy, “’Train surfers’: analysis of 23 cases of electrical burns caused by high tension railway overhead cables,” Burns, 26, 470-473 (2000)
·  Geiger, J. D., J. Newsted, R. A. Drongowski, and Lelli, J. L., Jr., “Car surfing: an underreported mechanism of serious injury in children and adolescents,” Journal of Pediatric Surgery, 36, No. 1, 232-234 (January 2001)
· Carey, J., M. C. McCarthy, A. P. Ekeh, L. Patterson, and R. Woods, “Car-surfing in southwest Ohio: Incidence and injuries,” Journal of Trauma. Injury, Infection, and Critical Care, 59, No. 3, 734-736 (September 2005)


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