SI Measurements Numbers are tools, not rules. G. O. Ashley

SI System The most common system of measurement around the world is known as the Systeme International d’Unites or SI system. You can think of it as a modernized metric system. There are two main reasons why it is so common: 1.The SI system uses a decimal system as its base. 2.The SI system has seven standard or base units and all other units are combinations of these seven. The U.S. is one of the few countries that has not switched over completely. However, when it comes to science we do use the metric system!

SI System Prefixes A prefix may be added to units to produce a multiple of the original unit. All multiples are integer powers of ten. For example, kilo- means a multiple of a thousand hence1 thousand millimeters =1 meter. Prefixes are never combined: a millionth of a kilogram is a milligram not a microkilogram.

SI System Prefixes PrefixMultipleScientific NotationAbbreviation mega-1,000,000 (Million)10 6 M kilo-1,000 (Thousand)10 3 k hecto-100 (Hundred)10 2 h deka-10 (Ten)10 1 da deci-0.1 (Tenth)10 -1 d centi-.01 (Hundredth)10 -2 c milli (Thousandth)10 -3 m micro (Millionth)10 -6 μ nano (Billionth)10 -9 n

Memorize the metric prefixes!

I Do: 492 km = ______ cm Which one is larger? 8.20 g or 45.8 cg? We Do: 1.2 mg = _______ g Circle the one that is correct: 7.52 L = mL or 3.4 cm = 34 mm You Do: 5.12 m = _______ dkm Circle the one that is correct: 59.0 mg = g or 0.82 kg = g

Mass Is NOT The Same As Weight!!!! Weight is a force that measures the pull on a given mass by gravity. Weight is different than mass, which is a measure of the quantity of matter. Weight can change depending on its location (that’s why astronauts weigh less on the moon- there’s less gravity), however mass is independent of location and remains constant. Objects can become weightless, but never massless.

Mass Is NOT The Same As Weight!!!!

Temperature Measurements The SI base unit for temperature is kelvins. Absolute zero is 0 K, a temperature where all molecular motion ceases to exist; it has not yet been attained, but scientists are within thousandths of a degree of 0 K. No degree sign is used for Kelvin temperatures. Even though the SI base unit for temperature is the Kelvin system, the Celsius scale still remains the most commonly used temperature scale for scientific work. Here are some formulas you might find useful: Celsius to Kelvin K = °C Kelvin to Celsius °C = K – 273 Fahrenheit to Celsius °C = (°F - 32) x 5/9 Celsius to Fahrenheit °F = (°C x 9/5) + 32

Celsius to Kelvin K = °C Kelvin to Celsius °C = K – 273 Fahrenheit to Celsius °C = (°F - 32) x 5/9 Celsius to Fahrenheit °F = (°C x 9/5) + 32 I Do: 79.2 F = ___________________________ K We Do: 68 °F = ____________________________ K You Do: 25 K = ____________________________ °F

Precision and Accuracy Precision – how well repeated measurements of a value agree with one another (Precision is a measure of exactness) Accuracy – is determined by the agreement between the measured quantity and the correct value. (Accuracy is a measure of rightness)

Precision and Accuracy

Which group(s) made precise measurements? Which group(s) made accurate measurements? (accepted value is 4.01 mL) Group123 Trial mL4.01 mL25.02 mL Trial mL4.02 mL25.01 mL Trial mL4.00 mL25.03 mL

Percent Error Percent Error-is calculated by subtracting the experimental value from the accepted value, then dividing the difference by the accepted value. Multiply this number by 100. Percent error = accepted value - experimental value X 100 accepted value

Percent Error Example: A student takes an object with an accepted mass of grams and masses it on his own balance. He records the mass of the object as g. What is his percent error? Use this formula: Percent error = accepted value - experimental value X 100 accepted value