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FEATURES: ASK RC: NEWTON'S LAW
Richard Cunningham
April 13, 2009
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The great Columbian climber Luis Herrera (R) knew how to use gravity to his advantage
(Photo: Yuzuru Sunada)
If Newton's law is correct, then gravity should act the same for a heavy or a lightweight rider. Theoretically, both riders should descend a hill at the same speed. Why is it then that my larger friends descend so effortlessly on their heavier bikes while I must pedal like mad just to stay on their wheels?
– Sarah G.
In a vacuum, and without factoring in friction and rolling resistance, your (I assume) skinny body and 13-pound carbon bike would descend at the same speed as your larger rivals because objects fall at the same rate, regardless of mass. Wind and mechanical friction, however, are the enemies of skinny cyclists, because less mass means that less energy (from gravitational force) is available to overcome friction in the form of wind and rolling resistance. In a vacuum, a feather and a steel marble fall at the same rate. In the atmosphere, as long we are traveling downward towards the center of the earth, mass always wins.
You are the feather in this equation and your big 45-mile-an-hour friends are – well you probably get the idea. My advice is to do what tiny Colombian climbing specialists do at the top of a mountain pass: sit up and wait for a couple of big guys with fresh legs to draft on the descents. You may have to work hard to stay on their wheels, but it's still the fastest way down the mountain. You can always return the favor on the next climb – gravity works both ways.
Contact Richard Cunningham for questions or comments, or just to talk bikes at: askRC@roadbikeaction.com
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