Why Mr. Walker Rolls Faster Than His Wife
I noticed in a recent issue of your excellent magazine (Letters, momentum #29) that a reader was wondering why it is that he always coasts downhill faster than his wife. The answer provided wasn’t quite correct and, on the off-hand chance that no other physicist has already put the matter straight, here is my explanation.
Galileo was of course right – gravity (g) acts equally on all bodies, exerting a force (F = g x Mass) on you, of which some fraction (depending on the slope angle) accelerates you downhill. You will accelerate according to Newton’s Law: F = Mass x acceleration. Equating those two terms, the masses cancel and everyone should experience the same acceleration. Right?
Well, in the real world there are lots of extra forces, like tire friction, ball bearings and probably most significantly, wind resistance. But unlike the gravitational pull, wind resistance isn’t dependent on your mass but (roughly) to the surface area you represent to the wind and the speed you are going. This ‘force’ has to be subtracted from the force of gravity (g x Mass) that is trying to make you go faster and faster. So two people of the same size will have roughly the same wind force term, but if one is heavier, the wind force will be less significant compared to the accelerating force (g x Mass). A truck barreling downhill is insignificantly affected by wind resistance.
Now, wouldn’t two people of different weight have different surface areas? This is true, but it doesn’t scale evenly – a person twice the mass (or volume) will have less than twice the surface area. So the heavier guy still wins out.
There are other factors, such as tire resistance (pump up those tires!) which might give the lighter person an edge but all things being equal, it pays to put on the extra pounds going downhill. Uphill you have to put in the extra work, since the energy needed to get to the summit is again proportional to mass.
Keep up the great work. The magazine is always a joy to read.
Hope this helps,
Gertjan Hofman,
Vancouver
Birds of a Feather…
We wanted to thank you for your great magazine. We love it! Because we sometimes miss an edition we thought we’d like to subscribe. In the last edition we absolutely loved the section about different rider species – how hilarious! We ourselves are transforming into different species all the time and could most certainly identify ourselves with many of the traits mentioned.
Anya & Zeus,
Vancouver
I just read Sheldon Brown’s piece on bike-a-thons and I must respectfully disagree with his viewpoint on several fronts.
I am a lifetime bicycle rider, mostly commuting shorter distances (10 miles RT each day) and an occasional long ride (20 miles or so) every now and then.
I also have Crohn’s disease, an auto-immune disease that profoundly affects the way my body breaks down food into nutrients. Crohn’s sufferers experience low weight, fatigue and frequent, urgent trips to the bathroom. There are over a million people in the United States who have Crohn’s or its related condition, ulcerative colitis. Crohn’s and UC patients often suffer in silence because these are not “sociable” diseases – not the sort of thing one talks about in polite company. There is also no cure for either disease. My lifelong struggle with Crohn’s has effectively excluded me from certain athletic and/or outdoor endeavors. I dreamed of going long distances on my bike but never felt I could do it alone.
Enter “Get Your Guts In Gear” (www.ibdride.org), the bike ride for Crohn’s and colitis.
GYGIG rides took place this year in Austin, TX; New York City and Seattle, WA. After seven months of training, preparation and fundraising, I recently completed the Puget Sound ride in early August. These three rides saw the participation of a total of 200 riders who came together to ride, socialize and connect with each other. GYGIG became a rolling support group for Crohn’s and colitis sufferers, their caregivers, families and friends. Many of the riders have Crohn’s or colitis and we encouraged each other to keep riding when things got tough. Every single participant finished the Seattle ride without mishaps, and all of us came away with a sense of accomplishment unlike any other.
The three rides raised awareness about these diseases in the cities where they took place; and also raised over half a million dollars, most of which will fund research to find a cure for Crohn’s and colitis.
Because I participated in GYGIG this year, I was empowered to speak more openly about life with Crohn’s and to advocate for others with the disease. I took up an ambitious training program that got me involved with my local bicycling club (Oregon Randonneurs, THE most welcoming and friendly bike club I have ever found), and helped me develop strategies for riding longer distances while living with a chronic illness and accomplishing my goals. GYGIG-Pacific NW gave me the opportunity to ride 141 miles on my bike, traveling through some of the prettiest country I’d ever seen in my life. I know I will want to do this ride again, and have already talked it up to friends of mine in the hopes they’ll want to do it with me next time.
Thanks to GYGIG, although I am no super-athlete I have become some kind of bicycle hero to my friends and family. They all ask when they can go on a ride with me. Bike-a-thons do not always have to be ABOUT bicycle riding in order to promote bicycle riding.
Beth Hamon,
Portland OR
bikelovejones.livejournal.com
Find more reader comments on Sheldon Brown’s ‘Thons Reconsidered at: www.momentumplanet.com
Continuing The ‘Car-Free’ Conversation
Prior to the 1850s the streets in many towns in the world were car-free, but definitively not vehicle-free! In Europe, for example, while many people walked, often carrying big bundles on their head, back or shoulders, many others pushed hand-carts up to six feet long, loaded with food or goods delivered to various stores and workshops. Some people rode horses while others used horse-drawn coaches of various sizes. Many people used sedan chairs, as coaches with two or four horses often couldn’t go through many streets due to the number of people and vehicles. Long distance travel between towns was done by stagecoach, used by all the classes of the society.
The classical literature of European and Asian countries is full of tales about the crowded streets of most sizeable towns. King Henri IV of France was stabbed to death when the royal coach was stuck in traffic in the Marais district. This is why, from the 18th century on, very wide boulevards were created in many towns to try to reduce congestion.
Car free days in Europe are the one day each year when most of the downtown area of many cities are closed to cars. However, practically every single city in Europe has at least one area where cars are either banned or severely restricted, not just one day but every single day of the year! Even major cities like London and Paris have them. The latter has a rather extensive pedestrian area known as Les Halles, Beaubourg and Montorgueil. There are pedestrian streets in other districts of Paris as well.
Quite a few people at the [Commercial] Drive car-free days were campaigning against freeways in Vancouver and pointed to Europe car-free areas as an example. I visibly infuriated a couple of them by mentioning that the car-free/car-restricted areas of Europe were only made possible by the construction of freeways well outside the downtown historical heart of many towns in the 1960s. One typical example is Bordeaux, France, the more than 2,300-year old city where I was born and raised.
I can remember the days when there was only one bridge to cross the wide and fast-flowing river on the left bank of which Bordeaux was built. On long weekends and during the summer holidays, when huge numbers of cars were coming down from as far as Paris on their way to Toulouse, the Pyrenees Mountains and Western Spain, it took several hours to cross that bridge.
Several bridges and a freeway around the town were built in the 1960s. Most of the traffic was thus directed away from downtown and this allowed for the creation of a pedestrian area several kilometres long that opened in 1975. That area was drastically expanded between 2000 and 2003 when the three lines of an LRT system were built, necessitating the permanent closure of many car lanes (some went from four lanes to one or even none).
Several cities in France use steel posts to block the entrance of the pedestrian streets. A swipe of a magnetic card lowers the posts in the ground allowing limited access to delivery vehicles (before stores opening the morning) to homeowners living in the area (early morning and late afternoon only) and of course to emergency vehicles (any time).
J-L Brussac,
Coquitlam, BC
