efficiencies ...

Roger sent an email reminding that a gallon of gas is equivalent to about 40 Kw-hrs of energy and a person riding a bike takes about 0.1 Kw of power. You can ride a bike for about 400 hours on the energy in a gallon of gasoline and your physical health (assuming you were able to ride safely) would probably be much better than if you followed a sedentary lifestyle - at the very least you would probably be thinner and look better.

Looking into my middenheap of numbers collected over the years I find several rules of thumb and real numbers (great things to have on the tip of your tongue for physics prelims):

• an average adult (age 18 - 65) can produce 3 watts/kg for more than an hour

• amateur athletes produce 5 watts/kg for several hours

• elite athletes produce 6 and sometimes 7 watts/kg for several hours

• on a flat road a 70 kg adult requires 100 watts to walk 5 km/hr (about 3mph). a bicycle rider requires the same energy expenditure to travel 25 km/hr

• standard biking is 1.6 kJ/(km∙kg)

• standard walking/jogging is 3.8 kJ/(km∙kg

In any event a 80 kg man with 5 kg of groceries in a 2000 kg SUV makes little sense when the store is a few km or less away.

Comments

[Alan Little]

These figures strike me as overoptimistic.

> an average adult (age 18 - 65) can produce 3 watts/kg for more than an hour
> amateur athletes produce 5 watts/kg for several hours

Not sure which of these I am. But in my 20s when I did a lot of rock climbing, some running and was fit, I could sustain 250 watts on a rowing ergometer for about 10 minutes. For "more than an hour" I could sustain about 150, or 2W/kg.

> elite athletes produce 6 and sometimes 7 watts/kg for several hours

For about half an hour. Somewhere in the low 200s total output is what the world's absolute top endurance athletes can keep up for several hours. Source: Tour de France 2005 diary by Floyd Landis' coach (Google "site:www.bicycling.com allen lim") Maybe marathon runners or ironman triathletes could do a bit more as a one-off with several weeks recovery, but not six days a week for three weeks like Tour riders

Both my figure and Lim's are power output at the crank/flywheel, not total energy output of the body incl. waste heat. Might that be the discrepancy?

> on a flat road a 70 kg adult requires 100 watts to walk 5 km/hr (about 3mph). a bicycle rider requires the same energy expenditure to travel 25 km/hr

Cycling at 25 kmh feels considerably harder than walking to me

> 80 kg man with 5 kg of groceries in a 2000 kg SUV makes little sense

I haven't owned a car for five years. The only time I routinely miss it is weekly family shopping, which is a great deal more than 5 kg and simply not feasible on a bike.