Momentum

“Bodies at rest tend to remain at rest, and bodies in motion tend to remain in motion, unless acted upon by an outside force.”  Isaac Newton’s First Law of Motion indirectly illustrates one important reason why the bicycle is one of the most efficient machines ever invented to augment human energy.  Both cyclists and runners spend considerable physical effort to put themselves into motion.  A good runner might run 15 miles an hour for a mile; an average cyclist could pace him easily using the mechanical advantage of gear ratios.

But when the runner stops moving his legs, he almost immediately stops.  If a cyclist pedaling alongside the runner stops moving his legs at the same time, he will keep on going for quite some distance.

Why?

Friction.  The effect of the runner putting his or her foot on the ground without further effort is an effective “brake,” because the contact between the two is strong, a literal “big footprint.”  The upper body wants to continue forward, and so the runner doesn’t literally stop “immediately.”  There are a few involuntary strides induced by that forward momentum, but the firmness of the foot’s contact with the ground soon halts the runner.

The cyclist, on the other hand, is on that magic machine.  Its contact with the ground is on two points the approximate size of a dime (for a road bike), and the wheels are rolling.   So when the cyclist stops pedaling there’s relatively little to prevent the momentum of the body and the bike from continuing for some time.  Eventually the friction of the bearings in the axles and the tires on the ground will stop the bike, but only after a long glide.

One way to conserve energy on a bike ride is to use this momentum to advantage.  It makes sense to build up speed on downhills so that momentum will help maintain speed on the uphill that follows.  Rolling very slowly through a safe “stop” area, or graciously accepting an automobile driver’s invitation to take the right of way, will save large amounts of energy as opposed to beginning again from a dead stop.  (Safety first, of course.)  A skilled rider can find dozens of ways to maximize the force of momentum on every single ride.  Three cheers for physics!

©Arnold J. Bradford, 2010.

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