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* To measure objects and substances we need both a number and a unit. Scientists use a set of measuring units. Scientist uses a set of measuring units call SI or international system of units, this system is built upon 7 metric units base units.
Table 1: SI base units
Quantity measured | Unit | Symbol |
Length | meter | m |
Mass | Kilogram | kg |
Time | Second | s|
Electric current | Ampere | A |
Temperature | Kelvin | k |
Amount of substance | Mole | mol |
Intensity of light | Candela | cd |

Table 2: derived units
Quantity measured | Unit | Symbol |
Area | Square meter | m² |
Velocity | Meter by second | m/s |
Force | Newtons | N= kg×m/s² |
Volume | Cubic meter | m³ |
Work | Joules | J= kg×m²/s² |
Density | Kilogram by cubicmeter | kg/m³ |

Table 3: prefixes
Multiply unit by | Prefixes | Symbol | Meaning |
1000000000 | Giga | G | 10⁹ billion |
1000000 | Mega | M | 10⁶ million |
1000 | Kilo | K | ₁
10³ thousand |
0.1 | Deci | d | 10¯ tenth |
0.01 | Centi | c | 10¯² hundreth |
0.001 | Mili | m | 10¯³ thousand |
0.000001 | Micro | | 10¯⁶ million |
0.000000001 | nano | | 10¯⁹ billion |

* Theexponent tells the number of places to move the decimal point. For a positive exponent the decimal is move to the right. For the negative exponent, the decimal is move to the left.

Conversion of unites
* The easiest way to convert units is to use conversion factors. We can use conversion factors in length, mass, volume and density. The units to which you are converting factors should beplace in the numerator. It’s useful to cross out units that cancel to help keep track of them.

1. Conversion factors for length
* Length is the distance between 2 points. The instruments to measure length are: metric rules, meterstiks, measuring tape.

2. Conversion factors for mass
* Mass is the amount of matter in an object. It is measured with an instrument calledvalance. Mass and weight are different concepts. Mass is the same everywhere. The weight changes depending on the place the object is. In fact the weight is the downward pull of an object due to gravity. If the pull of an object is strong the object wastes a lot.

3. Conversion factors for volume
* The amount of space that an object occupies. There are 2 ways to obtain the volume:
a) Forobjects that have well-defined shape such as a brick or ball

Formula: V= length × width × height V=L×W×H m³ or cm³
b) For objects with irregular shape we use the displacement method
1. Add water to a graduated cylinder, copy the level of water.
2. Submerge the irregular object since water and the object cannot occupy the same space, the water will move upward,read the new measurement and copy.
3. Subtract the initial volume of water from the final one and the answer will correspond to the objects volume.
c) For liquids: the volume corresponds to the space the liquid takes up in the container V L, mL 1mL= 1cm³, the instrument to measure volume are: graduate cylinders, pipets.

4. Conversion factors of temperature
* Temperature isthe measurement of now cold or hot an object is. To measure the temperature we use a thermometer, and a thermometer is an instrument like a glass cylinder tube with a bulb at one end. It contains a liquid inside that could be mercury or alcohol. The liquid rises when the temperature increases.
* SI UNIT: K H₂0 freezers at: 273k
* H₂0 boil at: 373k
* Zero absolute zero
*Thermometers that use “K” scale are inconvenient to use because they are too big. There before Celsius and Fahrenheit scale is more used.
Celsius: °C (or centigrade) Fahrenheit: °F
H₂0 freezers at: 0°C H₂0 boil at: 100°C H₂0 boil at: 32°F H₂0 freezers at: 212°F
5. Conversion factors for density...
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