## Thursday, October 13, 2011

### Speed Conversion and How Fast Is Light?

Have you ever thought about speed? What does it really mean? Well the boring academic version of what is speed goes along the following lines;

Speed (v) is defined as the magnitude of the velocity that is the derivative of the position with respect to time.

Makes perfect sense? No didn't think so. To understand speed and the various ways you can measure speed lets first take a small history lesson.

Around 1667 Galileo tried to determine the speed of light, remember there was no electrify then so Galileo was experimenting with lamps and having an assistant cover and uncover the lamps. Hence Galileo though the speed of light was 10 times faster than the speed of sound.

Ole Roemer a Danish astronomer tried to work out the speed of light while observing the moons of Jupiter, which btw Galileo had helped find. Anyway Roemer can to the conclusion that light travelled at 200,000 Km/s or Kilometres per second.

In the 1840's a French physicist, Louis Fizeau devised an experiment, with a spinning will with clogs on the wheel and a mirror 5 miles (about 8 Km) in the distance, (we will get to the abbreviations later); by reflecting light from the mirror, through the spinning wheel Louis determined the speed of light to be 313,300 Km/s

Another French physicist (must have be part of their University course), Leon Foucault in the 1860's tried much the same experiment as Fizeau and arrived at a figure of 299,796 Km/s for the speed of light.

What does all this tell us? Well for one thing measuring light from the 1660's through to the 1900's was not easy, but it also tells us that we; people have always tried to measure speed to find out how fast sometime is.

If we just side step for the moment at look at some of the abbreviations you may come across.

Firstly the standard MPH and Km/h. MPH is of course Miles per Hour and still used in both America and the UK. Km/h is Kilometres per Hour and is used in most countries that have converted to the metric system The Metric system used to measure, speed, weight etc. is based on tens. The metric system of measurement is used mainly in Europe, though countries like Australia adopted the system many years ago.

As an interesting point though, both systems still use the same measurement of time, there is no metric time.

KMs is Kilometres per Second, usually used to measure the speed of light. Other Units of measure for speed are:

Meters per Second (m/s), the metric version of Feet per Second.

Knots (knots); this is used by boats and ships and they usually talk of nautical miles which is different to a normal mile.

Feet per Second (ft/s); what you would use to measure, walking or running, also for some reason used for measuring braking distance for say a car.

Mach number; Mach numbers are the speed divided by the speed of sound, you here of this mainly with jet engines. The speed of sound is 1235 Km/h or 768 mph. But these figures will vary slightly depending on the air temperature and if you are at sea level or not.

So what have we learnt so far, well we know people had trouble measuring the speed of light, there are a number of different ways to display what the speed is and the speed of sound can change with the weather.

As an interesting foot note to the speed of light, the US National Bureau of standards lists the speed of light as being 299,792.4574 Km/s.

While the British National Physical Laboratory lists the speed of light to be 299,792.4590 Km/s.

Don't worry we can't see Who Wants to be a Millionaire asking what about the 2 sets of figures.

So back to the question of what is speed, well it's really how fast you are traveling at a particular time, be it by car, foot, or on a plane, and what did the "v" mean, well when the academics' talk of speed they usually call it velocity and that's where the v comes from.

As an experiment you can try and measure the speed of light, or just find the answer here which is:

299,772.458 Km/s. This was agreed upon in 1983 at a conference in Swiss land you can find out what that is in MPH by doing a quick search with your favourite search engine.

Jon Grey is an IT specialist with a strong interest in history and the scientific discoveries that have occurred without the computing power we have today.