A friend asked me to post this: when you are, say, striking metal on metal, sparks will appear, first in a straight line, then ending in a little starburst. Is there a name for that straight line?

I suspect it's a [modified] section of a parabola, so short that it's hard to differentiate it from a straight line.

A flying spark is a piece of incandescent metal or other matter ejected from some glowing object; we see its trajectory because it's hot enough to give off light. When/if at some point it bursts into pieces, still glowing, each piece is thrown out into its own path. After they cool off we don't see them, but [except for air resistance] they're all still just falling bodies with an initial velocity, and the paths are all parabolic. "S = 1/2 a t squared" or some variation of that.

[Of course air resistance can be considerable for objects that small, so the deviation from a true parabola may be quite significant.]


Edit: On rereading, I suspect you weren't asking for the name of the shape of the line at all...but I think the gist of what I said covers what you were getting at, so maybe I did answer the question after all.

Is there a name for that straight line? While the previous post demolished the idea that it was straight, I shall disprove that it is a line: We only see a line in this case because our visual perception is not fast enough to follow the flight of the luminous point. Do you still think we should name it?

>Do you still think we should name it?

How about a trailblaze? Those who illuminated us on the subject would then be trailblazers.

it's a [modified] section of a parabola

Yep, wofa, I was thinking "arc". I was also thinking about those childish depictions of shooting stars, where you have a (usually 5-point) star trailing several arcs.

I would expect there to be a word for this in firework display terminology. And with Bonfire Night rapidly drawing close over here, I know some people I can try asking...

Cool! Er, hot. ;-) Thanks, folks. Now--my friend wants to know if there's a name for the starburst, too, please.

Jackie, i'm unable to provide an answer to your friends question but it relates to an interesting topic called *spark testing*
In days of old *pre spectrophotometry* metallurgists and blacksmiths made use of these sparks and starbursts to provide an approximate chemical analysis of a metal subjected to sparking.
When abraded by a grinding wheel or abrasive disk each metal produces a shower of sparks. each element has it's own characteristic spark differing from others in size, shape, colour and star burst pattern. A well tained and experienced metalurgical eye could use this phnenonenom to provide you with the composition of that metal.
While not as precise as spectrophotometry many old hands swore by it.........and no doubt at it.

Hello, lapsus...

And I must ask: Why wouldn't this metallurgist have known the composition of the metal before producing the sparks?

Would a blacksmith pick up a piece of random metal that he was unsure of its composition, and then set it to sparking to learn more about its composition? If this was the case, then that's a very interesting analytical tool you've just told us about.

ll, I just grin every time I see you here! Thank you; and, how on earth do you know that? [impressed e] Was one of their purposess to test whether a particular piece was in fact what its purveyor purported it to be?

I used to do a lot of grinding in machine shop of a foundry. Iron and steel make the most
spectacular sparks, which vary according to composition of the metal being ground. My brother who is a
mechanical engineer told me he used to have books with pictures showing patterns
produced by different alloys, but said he did not recall any names having been
given to the various patterns observed.

*waving at lapsus linguae*

Don't know nothin' 'bout metalurgists, Dub Dub but a blacksmith would be called upon to work on all sorts of metals that he wouldn't know what they were (except in a very general sense). If you are repairing some old metal thing, or using scrap metal to make something new, it's handy to know what it is before trying to weld it to something else.

Is there a name for that straight line?

Yes, a tracer.