This past week, I committed an unforgiveable act of bicycling while answering my cell phone, in the rain, approaching
a green traffic light. Of course it turned to red before I noticed and I proceeded through the intersection. Timing was
such that the cars had not started to move, so I was not killed. I never run lights in any kind of traffic and this
has permanently modified my behavior very certainly. It’s easy to do things not commonly performed on a bicycle when
one’s bicycle is a velomobile, but I’ve removed cell phone use from the list.

How long does it take to honk?

One additional consideration in the dangerousness of cars versus bikes is the difference in the reaction time it takes for the operator of a car versus a bike to sound a warning. Depending on how habitual a honker one is, it may take more than a split second for a car driver to find and hit the car horn when about to bump a pedestrian.

Contrast the reaction time it takes for a cyclist to open their mouth and yell “Look out!” or “Aaaah!” There is only instinct involved in yelling, versus the learned response of hitting a horn, which in my driving experience can be slowed down by the more powerful learned reaction to hit the brakes hard. That difference in audible warning timing might be enough to avert some accidents. I think it saved my skin and that of the old lady who stepped in front of me on the bike path the other day. She heard me, stopped just in time as I swerved just in time, and we brushed by inches apart. If she’d stepped out from between parked cars and I’d been driving one, she wouldn’t have been able to hear me yell, and I couldn’t have stopped or swerved as fast as I did on the bike. Neither one of us would be very happy llamas right now.

I also survived a potential car collision on a bike once, because the dude cutting into my lane happened to be driving a convertible. If he’d had the windows up and the radio on, he wouln’t have heard my well-thought-out articulate warning of, “Hey, don’t cut over here!” which under duress came out as the aforesaid “Aaaah!!” Luckily that was sufficient, and I slipped by with a couple inches to spare.

This is part of why I never, ever wear headphones while bicycling. And always wear a helmet on my North American coconut.

I’m glad you enjoyed the explanation. For precision, let me elaborate a bit. The key concepts, in this as in all mechanics scenarios, are force and energy. Since we’re considering it, however, the definition of “jerk” is “the rate of change of acceleration”, not “a short-term acceleration”. If you haven’t had calculus, think of this as how fast the quantity called “acceleration” is changing at any moment. Also, note that acceleration is thought of similarly as how fast the quantity called “velocity” is changing at any moment. Likewise, velocity is thought of as how fast the quantity called “position” is changing at any moment. You probably have an intuitive kinesthetic understanding of position, velocity, and acceleration. There’s a reason why evolution hasn’t equipped us for perceiving or thinking about jerk. Tempting as it is to keep differentiating ad infinitum, our universe isn’t constructed so as to care about jerk and higher derivatives.

Some have defined “yank” to be mass multiplied by jerk (there’s probably a social commentary buried in that definition), so that these two quantities are related in the same way that force and acceleration (and for that matter, momentum and velocity) are related.

Your mention of helmets is appropriate. You’ve probably seen “one-time”, bicycle-style helmets, as distinguished from “reusable”, skate-style helmets. I guess the intent is that the former are more appropriate for the rare catastrophic collision, and the latter are designed for frequent lower-energy collisions. In theory, while both should have a much lower elastic modulus and elastic limit than bone, the one-time helmets might have a lower elastic limit than the reusable, which could allow them both to increase the “crush distance” and to dissipate more energy by breaking chemical bonds, at the expense of having to buy another helmet. However, they all appear upon examination to be made of polystyrene. I suspect that any such helmet is damaged somewhat by all impacts, and damaged beyond usefulness only rarely.

I too have been knocked unconcious while wearing a helmet. I fell from a horse while wearing a snowboarding helmet. That same helmet had earlier provided seemingly complete protection from a tree encountered at high velocity while snowboarding (helmet marketers would probably have something to say about this sequence of events, especially in contradiction to the last point of the previous paragraph). Maybe this winter I’ll buy a new helmet. If I knew I were going to have a terrible collision tomorrow, I’d probably buy a new helmet for the occasion.

As it is, I bike every day, and only rarely (e.g., on official rides that for some reason have insurance) do I wear a helmet. Last year I had a bike wreck that bent my fork enough to require replacement. I wasn’t wearing a helmet, I flipped over the handlebars, I landed on my lower back on asphalt, and I was sore for a couple of days. My head never touched anything so I suppose a helmet wouldn’t have mattered for this incident.

There is a set of collisions for which helmets increase the probability of survival. If we order all collisions by the energies involved, that set will fall in the middle of the continuum. If collision survival were my only value, I would wear a helmet all the time. Of course, in that case I probably wouldn’t ever operate any vehicle near any street. Some other, implicit values must be responsible both for my decision to bike and for my typical decision to do so helmetless. Those values are probably reinforced every time a driver realizes that I know she knows what a terribly unsafe maneuver she just made, and then rolls down the window and yells that I “ought to wear a helmet”. Possibly my value set isn’t consistent or rational, but I try to behave rationally in response to that value set.

Jess

That was great reading. I never took physics in college and I feel deprived for it, but I think I still have some
understanding of the sciences involved. Your explanation and elaboration was just as I have pictured such things in my
alleged mind. I took the terms “jerk” and “yank” to represent short duration accelerations in a direction not necessarily
desired by the designers of a ride, and as such, they seemed to be simpler ways of saying so. I’ve found such experiences
on a roller coaster to be unpleasant and the primary reason I don’t care to ride one. I do enjoy riding my bicycle, however
and hope not to collide with a jerk. Here in Florida, there are plenty of northern visitors, so I could possibly collide
with a yank.

On a more realistic note, my wife and I had the unfortunate experience (back in the early 80s) of a drafting overlap,
resulting in our combined mass on a tandem bike colliding with a tree. Very little elastic involved in the energy transfer,
but plenty of heat in the form of bent tubing and damaged tissues. Luckily the damage to the tissues was mostly caused
by scraping skin on the tree.

I have no intent of causing a helmet war, but the same accident noted above happened when bike helmets were not a common
sight. We were on a club ride and were the only riders with helmets. The tree was angled in such a way that my helmeted
head impacted first. I was knocked out, but came about a moment later. On succeeding club rides, there were six more
riders with helmets, and the one after that, a dozen or more. Now it’s unusual to see club rides without head protection.
In the spirit of the topic, I’m sure there was beneficial energy transfer between the tree and my helmet!!

fred

Jim may have been unclear in his description of this situation, but what you write is simply wrong. I normally wouldn’t enter the fray after more capable physicists have already withdrawn in disgust, but in this case the forces of ignorance threaten real harm to the cause of bicycling, so if I didn’t step in my own degree would be worth a bit less.

First, let’s back away from oddball physical terms that few engineers and even fewer physicists use. Let’s talk about energy, because in this case as in most others it’s much more instructive. We all know that the overall energy of a given system is conserved, but that it may be transferred among various components of the system. We know that the total kinetic energy of our two-component system is the sum of the respective mv^2 quantities. We know that as the pedestrian and vehicle tend more toward being two billiard balls, their collision will tend toward being more perfectly elastic. In the perfectly elastic case, the pedestrian would fly away at a velocity determined by the ratio of her mass to that of the vehicle, the vehicle would effectively stop, and we could say that the vehicle had transferred all of its energy to the pedestrian. We don’t live in that world, so much of the energy goes somewhere else. While the elastic modulus of billiard balls implies strains far below their elastic limit at the stresses associated with customary billiards energy levels, the elastic moduli of mammalian tissues make no such implication for the energy of a speeding automobile.

As one of the other posters said, some airbags are triggered by collisions with deer. The mass of a typical deer approximates that of a typical human, so we’ll take this as proof that the vehicle transfers some kinetic energy to the pedestrian even in the real world case where the vehicle is a car. If you believed Newton you wouldn’t need that proof. As an aside, what do you think an airbag sensor measures? Acceleration or jerk?

So now we have that the vehicle has transferred energy to the pedestrian, much of which does not end up in kinetic form. Where did the energy go? Recall that energy and work are the same quantity. Somehow, a force has acted over a distance. The distance isn’t far (although it’s farther than that for the billiard ball case, it’s shorter than that for an insect, splat), and the force is great. At car-on-highway energies, this force will certainly be great enough to strain and then break bones, ligaments, membranes, and other structural and vital tissue. Bicycle energies are lower, the distance in contact will be longer, the force applied in contact will be less, the stress to the vehicle (or at least the bike rider) will be more similar to that to the pedestrian and both stresses will be less, and, on average, the likelihood of exceeding the elastic limit of a fatal proportion of vital tissue will be less. The sad news from New Zealand is a reminder that that isn’t always the case. Human impact resistance is probably somewhat chaotic at the margin. It’s certainly possible for one human body to cause the death of another without the use of a bicycle or any other dangerous tool.

Where does the energy go next? It breaks some chemical bonds and then meets its destiny as heat.

In the foregoing I fudged a bit because I was talking about car-on-highway energies. If the hypothetical car and hypothetical bicycle strike the pedestrian with the same velocity, what can we predict? Since the car has much greater mass (thus, even at the same velocity, greater momentum and greater energy) than the bicycle and rider, the somewhat inelastic collision with the pedestrian is unlikely to stop the car, while the bicycle will cease movement entirely. Therefore in addition to the forces of impact, you must consider the forces applied by each tire in series, unless the driver of the car notices the collision and applies the brakes in a timely fashion. Even if we assume that a typical pedestrian will be thrown clear of continued peril (after all, the collision is somewhat elastic), the car will still cause more damage at the moment of collision. The steel front end of a typical automobile has a much higher elastic modulus than tissue, so over a shorter contact time and distance it will spring back and deliver all potential energy absorbed in deformation to the body of the pedestrian. This means more force will be applied, which is what actually strains and damages tissue. This is why punching a steel post hurts more than punching a jerk, and this is the advantage of the crumple effect that certain automobiles employ – the energy is dissipated in crumpling the vehicle rather than the pedestrian, which is certainly a positive development. Consider that because of elasticity the pedestrian will be travelling faster after the collision with the car than after that with the bicycle, and so will be more vulnerable to secondary injury upon “landing”. Also consider the fact that (in my experience) in most collisions the bicyclist continues in motion after the bicycle stops, so there is actually a lower percentage of the bicycle’s already lower energy transferred to the pedestrian. I would rather be hit by a bicycle than by a same-velocity car in every case short of mounting a trampoline on the front of the car and arranging another one down the road a bit to catch me (which could be somewhat thrilling actually).

OK, enough real physics. You quote omnipotent wikipedia thus: “Jerk is used at times in engineering, especially when building roller coasters. Some precision or fragile objects such as passengers, who need time to sense stress changes and adjust their muscle tension, or suffer, e.g., whiplash can be safely subjected not only to a maximum acceleration, but also to a maximum jerk.”

You’ll note that even in the entry for jerk, in which jerk is the concept under consideration, even in the context of this somewhat contrived example the writer mentions maximum acceleration first before describing maximum jerk. Acceleration and force govern tissue integrity as already described. If I had to guess what jerk and yank (I’m not making that up!) are doing in this situation, I’d hazard that they affect how quickly a passenger must adjust the forces applied by her muscles to changes in acceleration so that her head doesn’t bounce from side to side, which could certainly annoy a roller coaster passenger. You’ve mistaken something that must be measured only in particular physical contexts for a deep meaningful concept.

The other examples you mention are identical to the vehicle striking the pedestrian, under a suitable change of inertial frame of reference, so I won’t address those, other than to say that there’s a reason they call the gas pedal the accelerator. I was trying to figure out how someone with such a superficial understanding of physics could lecture a postdoc in such a rude fashion, so I decided to examine “Resnick and Halliday”. I found the review by Lydia Joyce on this page: http://www.amazon.com/gp/product/0471232319 quite instructive. Wrote Ms. Joyce, “The moral of the story is that Halliday & Resnick is easy–and deceptive. For an extraordinarily superficial survey for someone with little to no background in physics or calculus, it is adequate. But if you want to understand at a deeper level what is happening and why–go somewhere else.” I guess I need wonder no longer.

The take-home for the bicycle enthusiasts who have witnessed this unfortunate exchange is that bicycles travel with much lower energies than automobiles. For a variety of reasons, the dissipation of these energies in a collision with a pedestrian is thus less likely to deform tissue in a fatal fashion. Add to this the greater situational awareness of a bicyclist over a driver, and I firmly believe that every able-bodied person who bikes rather than drives makes our society safer.

I just got my Free Radical, so I’ll be travelling with a bit more momentum from now on. b^)

I don’t doubt that cyclists can and do kill pedestrians, as the unfortunate Mrs Sugiyama proves. But absent any evidence that handlebars to the torso cause organ trauma leading to death, I’m still not convinced that being hit by a well-designed car is better (for the victim) than being hit by a bicycle. I’m with you on not wanting to find out what it feels like to be hit by a cyclist in the absolute worst-case scenario. But I’m willing to bet that, on average, a head-on impact from a bicycle causes less damage than a glancing impact from a car.

(Insert something clever about llamas here.)

“Cyclist Charged Over Death
A cyclist involved in a collision with a jogger in a Hamilton underpass in February will be
charged with careless use of a vehicle causing death. Police have been investigating the
accident for the past three months.
Momoe Sugiyama, 52, was jogging out of the eatern side of the tunnel at the corner of
Wairere Drive and River Rd on February 9 when she was knocked down by a man on a bicycle
and later died in hospital. The cyclist stayed with her after the accident”

So I’m with Eric, and wouldn’t want a set of handlebars in my guts at 30kph!
Shit happens, and it’s sad to see it be fatal.

Have fun and be careful out there!

Regards
Ted Howard
Nelson, New Zealand

The writers have been sacked� and the sackers have been sacked as well�

Blue…no yellowwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwww

You don’t understand it correctly. Check http://en.wikipedia.org/wiki/Jerk. I don’t know how you missed it, since you’re claiming an advanced degree in the field. A little embarassing, don’t you think? I even told you where you could look it up, if your textbooks weren’t handy.

Check it:

“Jerk is used at times in engineering, especially when building roller coasters. Some precision or fragile objectsââ�¬â��such as passengers, who need time to sense stress changes and adjust their muscle tension, or suffer, e.g., whiplashââ�¬â��can be safely subjected not only to a maximum acceleration, but also to a maximum jerk. Jerk may be considered when the excitation of vibrations is a concern. A device which measures jerk is called a “jerkmeter.”"

Thing is, my jerkmeter is TOTALLY MAXED OUT for this conversation.

You ask me to consider the case of a bicyclist riding at 20 mph crashing into a brick wall. This causes the acceleration of the bicyclist to go from 0, constant cruising speed, to extremely high, whatever it takes to bring the test subject to a halt while displacing the brick wall essentially none. Get that? A change from no acceleration to a large amount, over a small period of time, aka jerk, the third derivative of position with respect to time.

“It is this huge acceleration/force/momentum transfer rate that does the damage” great. It’s called jerk. Now you know, and don’t have to wave your hands around the concept because you have a name for it. No. Big. Deal. “transfer rate” aka “change in respect to time”. The amount of momentum or force which is transferred over a given time period is represented as jerk, a vector quantity. It’s the difference between shoving someone and punching them in the face; the latter is more of a jerk move, even when equally forceful.

I can demonstrate, suggestively, that it is jerk and not acceleration that causes damage in the typical case. Lets say you freefall for five seconds, at earth gravity. You agree that it would take five seconds of freefall at one G in the other direction to stop you? How about one second at 5 g? half a second at 10 g? The human body can take 10 gs of acceleration for two minutes or more, and you take at least that half a second to fully impact with the ground. But jumping from 0 change in acceleration to “a lot” in such a short time is enough jerk to kill you dead. The sensation of acceleration is smooth; the sensation of change in acceleration is, well, it’s a jerk.

I encountered jerk in Resnick and Halliday but you can read about it on wikipedia, which is an excellent resource which I wouldn’t have to lend you. You picked the wrong bloke to patronize; your suggestive bloviation about getting up out of a chair and pressing the gas pedal would evaporate under hard numeric analysis, as it is for instance jerk which causes whiplash. That little jerk you feel when you go from resting comfortably in your seat to pressed back into it: that little jerk, magnified, is what kills you in a crash.

This sentence represents a joke about llamas, showing how I’m way cool and above all this physics stuff.