Section 2.1 - Units of Measurement

In 1795, French scientists adopted a measurement system comprised of standard units called the metric system.

In 1960 an international system of scientists met to update the metric system – this revised system is called Systeme Internationale d’Unites or SI

Base Units

Base unit – a defined unit in a system of measurement that is based on an object or event in the physical world.

There are 7 SI base units

Prefixes

The following prefixes are used with all SI base units to make them larger or smaller by factors of 10

Time

The SI base unit for time is the second (s)

The frequency of microwave radiation given off by a cesium-133 atom is used to determine the length of a second.

Length

The meter (m) is the base unit for length.

One meter is the distance that light travels through a vacuum in 1/299,792,458 of a second

One meter is a little longer than a yard, and measures the first dimension - length

Mass

The SI base unit for mass is the kilogram (kg).

A kilogram is about 2.2 pounds.

The kilogram is defined by the platinum-iridium metal cylinder shown at the right.

The cylinder is stored in a triple bell jar to keep air out.

Derived Units

A derived unit is one that is defined by a two or more base units – for example speed (m/s) involves two base units.

Other quantities that are measured in derived units are area, volume and density.

Area and Volume

Area is a two dimensional measurement (length x width).

Volume is defined as the space occupied by an object.

The derived unit for volume is a three dimensional measurement (length x width x height) called a cubic meter (m3)

Volume

Another derived unit for volume is the cubic centimeter (cm3)

Volume for Liquids or Irregular Solids

The liter (L) is the SI unit for volume and is equal to 1 dm3.

Liters are used to measure the amount of liquid in a container.

One liter has about the same volume as one quart.

Smaller quantities are measured in milliliters (mL)

1 L = 1,000 mL 1 mL = 1 cm3

Density

Density is the ratio that compares the mass of an object to its volume.

Units for density are often grams per cubic centimeter (g/cm3).

Density = mass/volume

Temperature

Two temperature scales are used by scientists:

Celsius scale – devised by Anders Celsius who defined the freezing point of water as 0˚and the boiling point as 100˚.

Kelvin scale – devised by Lord Kelvin. The kelvin (k) is the SI base unit for temperature.

K = C + 273

Section 2.2 – Scientific Notation and Dimensional Analysis

Scientific notation expresses numbers as a multiple of two factors:

1) a number between 1 and 102) ten raised to a power, or exponent

Dimensional Analysis

Dimensional analysis is a method of problem-solving using conversion factors and focuses on the units used to describe matter.

A conversion factor is a ratio of equivalent values used to express the same quantity in different units. A conversion factor is always equal to one.

Example: How many seconds are there in 2 years?

Accuracy and Precision

Accuracy refers to how close a measured value is to an accepted value.

Precision refers to how close a series of measurements are to one another.

Percent Error

Percent error is the ration of a error to an accepted value.

For percent error