Essential Concepts: Measurements

Obj 2.1 Name the metric units of length, mass, time and volume and other common measurements

Obj 2.2 State the difference between the mass and weight of an object

1. Weight: Definition: the measure of gravity between two objects (one the objects is usually the Earth). Unit: Newton (N)

Because the force of gravity between two objects changes with the distance between them, an object's weight varies from place to place (Death Valley vs Mt. Everest); not good for science.

2. Mass: Definition: a measure of the quantity of matter. Units: the SI unit is the Kilogram (kg), but a gram (g) is often used as standard as well.

Mass is determined by comparing the mass to a standard using a balance. Mass measurements will be the same anywhere, and are thus useful in science.

3. Length: Definition: the distance between two points. Unit: the meter (m)

4. Time: Definition: interval between two occurrences. Unit: the second (s or sec)

5. Temperature: Definition: the average kinetic energy of the particles in an object. Units: The SI unit is the Kelvin (K) based on "absolute zero," or -273C. Another often used unit is the Celsius degree (C).

Conversions: K = C + 273. Note there is no degree symbol () with K.

6. Quantity: Definition: the number of things. Unit: mole (mol): 6.022 x 1023 "things." (Similar idea to a dozen being 12 "things.")

Other (not SI) metric units often used

7. Volume: Definition: the space occupied. Unit: liter (L) or milliliter (mL). 1 mL = 1 cm3

8. Density: Definition: the ration of an objects mass to its volume. Unit: g/cm3 Obj 2.3 Use metric prefixes

Metric prefixes (memorize those in italics) Prefix Symbol Meaning Example giga- G 1 000 000 000 (109) units 64 gigabyte (GB) hard drive mega- M 1 000 000 (106) units 233 megahertz (MHz) kilo- k 1 000 (103) units 1 kilometer (km) hecto- h 100 (102) units (not much used) deka- dk 10 (101) units (not much used) unit 1 (100) unit 1 meter (1m), 1 gram (1g) deci- d 0.1 (10-1) unit decimeter (dm) centi- c 0.01 (10-2) unit centimeter (cm) milli- m 0.001 (10-3) unit millimeter (mm) micro-  ("mew") 0.000 001 (10-6) unit micrometer (mm) nano- n 0.000 000 001 (10-9) unit nanosecond (ns) pico- p 0.000 000 000 001 (10-12) un picometer (pm)

Obj 2.4 Distinguish between the accuracy and precision of a measurement

Accuracy: how close a measurement it to the true value.

Precision: how close measurements are to one another. Note: precise measurements are not always accurate and vice versa.

If the true value is 23 mm, the measurements 21 mm, 24 mm, 20 mm, 25 mm are accurate but not very precise; while the measurements 18 mm, 19 mm, 17 mm, 18 mm are precise but not accurate. The measurements 22mm, 24 mm, 23 mm are both accurate and precise.