Re: [Phys-l] ivory windmills

[brian whatcott <betwys1@sbcglobal.net>]

If a pound of oil provides 5.3E7 joules per kg and the metabolic rate of 
a resting cyclist is 150 watts, and the peak rate is 360 watts, so that 
riding disposes of 210 watts,
how long will it take to dissipate 5.3E7 joules at a rate of 210 watts?
That would be 5.3E7/210 seconds, I imagine;  possibly 70 hours?

But John considers a student is embarrassed by the significant digits 
shown in the answer he gives rather than the orders of magnitude between 
estimates?
I know, I know - he was using a poetic turn of phrase - but nit-picking 
is SUCH fun!   :-)

B

On 4/12/2012 11:49 AM, John Mallinckrodt wrote:
> That's certainly what I would say.  Yesterday I took my students to task for giving answers like "1,911,741 years" to the question "How long would you have to pedal a bike at full throttle to lose 1 kg assuming that you generate 1/2 hp of useful power and your body is about 25% efficient at converting stored energy to useful work."  I don't feel the least bit guilty about having done so.  Quite the contrary, I would feel remiss in my instructional duties to allow them to continue embarrassing themselves in public that way.
>
> John Mallinckrodt
> Cal Poly Pomona
>
> On Apr 12, 2012, at 9:11 AM, Robert Cohen wrote:
>
> > I don't disagree with the main point but it seems to me that the sig fig examples serve to illustrate the problems with rounding intermediate steps, not the use of sig figs at the end.  Am I mistaken?

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