Why do we use the metric system in science?

1. Introduction to Physical ScienceMonday, Wednesday, ThursdayTom Burbinetomburbine@astro.umass.edu

2. Why do we use the metric system in science?

3. Remember • A system based on multiples of 10 is much more intuitive for humans • We are born with 10 fingers and toes • The math system that we use is based on 10,

4. Metric System • Any system of measurement needs three fundamental units • Length - meter • Mass - kilogram • Time - second

5. 1 kilometer = 1,000 meters • 1 meter = 100 centimeters • 1 centimeter = 10 millimeters • 1 kilogram = 1,000 grams

6. Things you need to know because we will use the metric system • one kilometer is 5/8 of a mile • one meter is 3.28 feet • one centimeter is 0.39 inches • 1 kg (mass) is equivalent to 2.2 pounds (force) on Earth • We will use the metric system in this class • Does anybody remember the Mars Climate Orbiter?

7. Mars Climate Orbiter • Software calculated forces for the thrusters in English units (pounds). • People controlling the spacecraft thought the calculated forces were in Newtons (metric). (One English pound of force equals 4.45 Newtons.) • Changes made to the spacecraft's trajectory were actually 4.45 times greater than what the JPL navigation team believed. • The spacecraft missed its intended 140 - 150 km altitude above Mars during orbit insertion, instead entering the Martian atmosphere at about 57 km. • The spacecraft was destroyed http://en.wikipedia.org/wiki/Image:Mars_Climate_Orbiter_2.jpg

8. Exercise #2 • 1 kilometer = 1,000 meters • 1 meter = 100 centimeters • 1 centimeter = 10 millimeters • 1 kilogram = 1,000 grams • one kilometer is 5/8 of a mile • one meter is 3.28 feet • 1 kg (mass) is equivalent to 2.2 pounds (force) on Earth

9. Meter • How is the meter defined?

10. Meter • Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole • International Prototype Meter was defined as the distance between two lines on a standard bar composed of an alloy of ninety percent platinum and ten percent iridium, measured at the melting point of ice.

11. Meter • Now defined as equal to the distance travelled by light in vacuum during a time interval of 1⁄299,792,458 of a second.

12. Gram and Kilogram • How are the gram and kilogram defined?

13. Gram and Kilogram • How are the gram and kilogram defined?

14. Gram and Kilogram • A gram was first decreed to be equal to “the absolute weight of a volume of water equal to the cube of the hundredth part of the meter, at the temperature of melting ice.” • Now, the International Prototype Kilogram is used as the standard. • It is made of a platinum alloy known as “Pt‑10Ir”, which is 90% platinum and 10% iridium

15. International Prototype Kilogram • Photo of Danish national kilogram prototype

16. Second • The solar day was divided into 24 hours, each of which contained 60 minutes of 60 seconds each • So the second was 1⁄86 400 of the mean solar day. • Now defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.

17. Any Questions?