The Language of Physics: Physical Quantities and Units

"The measurements of physical quantities are expressed in terms of units, which are standardized values." "All physical quantities in the International System of Units (SI) are expressed in terms of combinations of seven **fundamental physical** units, which are units for: length, mass, time, electric current, temperature, amount of a substance, and luminous intensity." "Since the derived units can be expressed as algebraic combinations of the base units, they can only be as accurate and precise as the base units from which they are derived." "A **conversion factor** is a ratio expressing how many of one unit are equal to another unit. A conversion factor is simply a fraction which equals 1." "Notice also that, although you can multiply and divide units algebraically, you cannot add or subtract different units. An expression like *10 km + 5 kg* makes no sense." "**Accuracy** is how close a measurement is to the correct value for that measurement." "**Precision** states how well repeated measurements of something generate the same or similar results." "The rule is that the last digit written down in a measurement is the first digit with some uncertainty." "For multiplication and division: The answer should have the same number of significant figures as the starting value with the fewest significant figures." "For addition and subtraction: The answer should have the same number places (e.g. tens place, ones place, tenths place, etc.) as the least-precise starting value." "The horizontal axis, or *x*-axis, shows the **independent variable**, which is the variable that is controlled or manipulated... Distance from the station is the dependent variable and should be plotted on the *y*-axis." "The four fundamental units we will use in this textbook are the meter (for length), the kilogram (for mass), the second (for time), and the ampere (for electric current)."