# International System of Units

## Discusses two different systems of measurement and how to convert between them.

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Practice International System of Units

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Strontium Clock

### Strontium Clock

Credit: Batholith
Source: http://commons.wikimedia.org/wiki/File:Alarm_Clocks_20101107a.jpg

If your alarm clock gained a second a day, you probably wouldn’t notice. It would take two months for it to gain a full minute. By scientific standards, however, your clock would be far too inaccurate. It would also be too inaccurate to time elite athletes, where races may be won by milliseconds. How can a clock be made that accurate?

#### Amazing but True!

• Strontium is the answer. Strontium is an alkaline Earth metal in group II of the periodic table.
• Like atoms of other alkaline Earth metals, atoms of strontium vibrate when struck by light. The vibrations occur millions of times each second.
• Credit: Sebastian Blatt, JILA, University of Colorado
Source: http://en.wikipedia.org/wiki/File:JILA%27s_strontium_optical_atomic_clock.jpg

A blue laser is shined onto ultracold strontium atoms, producing the fluorescence [Figure2]

• The unit of time called the second is already the most accurate of the basic units in the International System of Units (SI). With strontium clocks, the second can be measured even more accurately.
• Watch this video to see how a strontium clock works and why such accurate time keeping is important: http://news.discovery.com/tech/videos/new-atomic-clock-redefines-time-video.htm

#### Show What You Know

1. Compare the new strontium clock with the earlier cesium clock. How much more accurate is the strontium clock? Why?
2. Why are such accurate measurements of time important in science? Describe one example.
3. Why is extremely accurate time keeping important for the Global Positioning System (GPS)?
4. What are some other uses of strontium?
5. Strontium is never found alone in nature. Why?

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