# World Solar Challenge update for May

The infographic above (fifth in the series, and just a few days early) shows solar car teams that are likely to be entering in the World Solar Challenge this October, with my estimate of reported current progress (on a red–amber–green scale), taking into account recent social media updates. The team list has also been updated, and has a simpler traffic-light version ( ) of these estimates, together with more detailed news.

As well as building a new car, Principia are gearing up to race an old one at the FSGP in July, so they must be rather busy. Bochum continues to blog in German, and report that they are using plexiglas for the windows of their new car. Nuon have announced that their new car will have an autopilot (OK, that last one was an April Fool’s joke).

In general, teams should be well into construction by now. For some teams, I have seen no evidence that this is the case (which may simply mean that they are too busy to post). However, best of luck to all the teams!

# Mathematics of the Harp

After my second harp post, I thought I’d keep going with some mathematics. In particular I want to answer the question: why does a harp have that shape?

A modern electric lever harp (photo: Athy)

The physics of vibrating strings gives us Mersenne’s laws, which tell us that the frequency of a string of length L is (1 / 2L) √ T / μ , where T is the tension force on the string (in newtons), and μ is the density per unit length (in kg per metre).

For a string of diameter d and density ρ, we can calculate μ = A ρ, where A = π (d / 2)2 is the cross-sectional area. Nylon has a density ρ of 1150 kg/m3. For a nylon string of 1 mm diameter, we get μ = 0.0009 kg/m. Putting 0.448 m (17.6 inches) of that string under a tension of 140 newtons (31.5 pounds force), we get a frequency of (1 / 0.896) √ 140 / 0.0009  = 440 Hz. That is, the string plays the note A.

The important thing here is that the frequency is inversely proportional to the length. Over an octave the frequency doubles, which means that the string length halves. A 36-string harp covers 5 octaves, therefore if all the strings were made of the same material under the same tension, the longest string would be 32 times the length of the shortest. Doing some calculations in R for strings of diameter 0.8 mm under a tension of 140 newtons (31.5 pounds), we would get the following harp, which has strings ranging from 7 to 224 cm in length (note that the strings run from A to A, and the C strings are red):

Image produced in R. Click to zoom.

You can see quite clearly that, starting at the treble end, there is an exponential growth in string length. That makes for a terribly unwieldy instrument, and creates all sorts of problems in playing. In practice, we make bass strings thicker (and usually of heavier material) and we vary the tension as well – although, to make life easier for the harpist, we want the properties of the strings to vary reasonably smoothly. If we re-do our calculations with string diameters varying linearly from 3 mm to 0.6 mm, and tension varying linearly from 210 newtons at the bass end to 60 newtons at the treble end, we get a much more realistic-looking harp:

Image produced in R. Click to zoom.

We can flatten out the curve at the top a little by changing the way we vary the strings (after all, a guitar manages to span 2 octaves with all the strings being the same length). However, we cannot eliminate that curve completely – it is the inevitable result of spanning so many octaves, combined with the mathematics of exponential growth.

Left: an exponential curve (red) and a similar polynomial (dashed). Right: the quotient of the two functions (green) compared to a straight line (grey). Click images to zoom.

Mathematically speaking, the modifications to the strings have the effect of dividing an exponential function by some kind of polynomial (as shown above). Over a short range of x values, we can find a polynomial that fits the exponential well, and gives us strings of the same length. Over a wide range of x values, however, the exponential wins out. Furthermore, exponential growth is initially slow (sub-linear), so that (starting at the right of the harp), growth in string length is slower than the linear shift provided by the sloping base, which means that the top of the harp curves down. After a few octaves, growth in string length speeds up, and so the top of the harp curves up again.

A similar situation arises with the strings of a piano, although these are usually hidden from view:

Inside of a piano (photo: Alexandre Eggert)

And to finish, here is one of my favourite classical harpists in action:

# Calendar for May

Above, the calendar for next month (click for hi-res image). I have done calendars for:

# Marching for Science #7

Interesting summary of the Science March from STAT:

• Yes, it was a partisan anti-Trump event – “critics of the march who worried that it could turn scientists into an interest group to be isolated and ignored will likely feel their concerns validated after the event.”
• It was mostly white – “There were [speakers] who were immigrants, trans, gay, Native American, black, Latino, young, and old. … But that audience itself was largely white.”
• Industry science wasn’t there – “companies that are now marketing their ‘bold’ work in scientific discovery and developing new treatments largely lacked an official presence at the marches.”
• People had fun – “lots of kids, dogs, and people dressed as dinosaurs. … and plenty of off-rhythm dancing to funk bands.”
• What comes next is uncertain – “Will the march make a difference? Or will it end up as a historical footnote?”

March for Science, Washington, DC (photo: Becker1999)

# A belated calendar

Above, a belated calendar for this month (click for hi-res image). I have done calendars for:

# Misquotes for Science

It’s a tough call, but the award for silliest statement at the March for Science has to go to the line “Dante said that the hottest places in Hell are reserved for those who remain neutral in times of moral crisis.” Dante never said anything of the sort, of course – the line is derived from something JFK said (“The hottest places in Hell are reserved for those who in time of moral crisis preserve their neutrality”), derived in turn from a chain of misquotes going back to Theodore Roosevelt. I’ve written before about Dante and Science, but suffice to say that in Dante’s Inferno, the worst regions are actually icy cold, and “neutrals” are not found there:

Please, let’s not have any “alternative facts” about Dante. The climate of the Inferno is important too.

# Marching for Science #5

Further to my previous comments on the Science March, the graph below shows the (somewhat dubious) attendance estimates from Wikipedia for various cities (excluding vague counts like “thousands”), compared to the power-law predictor 0.47 D1.49 P0.78, where D is the fraction of the relevant state voting for Clinton last year (from Wikipedia), and P is the city population (also from Wikipedia).

The population P predicts 56% of the variance in turnout (not surprisingly), and D an additional 7%. Both factors were significant (p = 0.000000055 and p = 0.014 respectively). Prediction could probably be improved by using metro area population numbers for the cities, by using metro area election results (rather that state results), and by adding factors indicating the number of other marches in the relevant state (Colorado Springs, for example, was rather overshadowed by Denver) and the presence of universities (Ann Arbor, for example, is a university town). But the basic messages seem to be: Democrat voters do not like Donald Trump and Large cities attract large crowds. It would be interesting to compare the numbers here against other recent political marches which focused on different issues.