1. Units and Measurement

2. – here the unit is meters (m)
Units
5 m
– here the unit is meters (m)
Units tell what a number represents. The units are usually represented by a one or two letter abbreviation that follows the number. Most numbers in Physics should be accompanied by a corresponding unit, though there are some numbers that will not have any units.

3. Why are units important?
5 inches
is very different from
5 feet.
Units are important because a unitless number is not very helpful most of the time. If you were told to go to the hardware store and buy 5 of rope you might come back with 5 inches, 5 feet or even 5 packages. Without the units you wouldn’t know which one was needed. However if you were told to buy 5 yards of rope you would come back with the same amount of rope no matter where you bought it.

4. Why do we need standard units?
Would someone know how long 5 knuckles was?
If everyone tried to establish their own units of measurements then accurate measurement would be impossible. If you decided to measure in “knuckles” which is the length from the tip to the the knuckle of your thumb you would have to explain that to everyone you meet. A universal standard allows fast and easy communication of measurements.

5. The SI basic units -mks Length - meter Mass – kilogram Time - second
The MKS system is the basic system we will use in class. It is the most commonly used system in Physics.

6. The SI basic units - cgs Length - centimeter Mass – gram Time - second
The cgs system is used when dealing with smaller quantities. It is not as commonly used in Physics as the MKS system.

7. English basic units Length – foot Mass – slug Time - second
The English system is commonly used in the United States. It is not a decimal system and is not commonly used in the world as a whole, so it will not be used in this class. Note – the slug is the unit of mass; the more commonly used unit is the pound which is a unit of weight.

8. Metric Prefixes - large
mega M 106
kilo k 103
These prefixes indicate how many powers of 10 a unit represents. Thus a kilometer represents 103 meters or a thousand meters. The major advantage of the metric system is that it is a decimal system, which means all the units are related by powers of ten. Contrast this with the English system where one foot is twelve inches or a mile is 1760 yards.

9. Metric Prefixes - small
centi c
milli m
micro m
nano n