- 1. Unit and Measurement Topic 1
2. Lecture Outline
3. Physics and measurement
- Physics attempts to describe nature in an objective way through
measurement
- Measurement are expressed in units;
- Officially accepted unit are called standard unit
-
- Metric cgs system centimeter, gram, second
-
- British (used by the U.S.) feet, pound, second
4. Basic units and quantity
- Length, mass and time are the fundamental quantities;
combination of them will form all the other unit.
- Today the most important system is theSyst me International
(SI), which also based on the metric system meter, kilogram and
second
5. Length
- SI unit of length: meter (m)
- The original definition: one ten-millionth of the distance from
the earth equator to either pole
- The newest definition: The meter is the length of path traveled
by light in vacuum during time interval of 1/299,792,458 of a
second
6. Time
- SI unit of Time: second (s)
- The original definition: one second is define as 1/86,400 of a
mean solar day (24h/day 60min/h 60s/min = 86,400s/day)
- The newest definition: Time required for 9,192,631,770 periods
of radiation emitted by cesium atoms
7. Mass
- SI unit of mass: kilogram (kg)
- The original definition: one kilogram is the mass of 0.10 m 3of
water
- The newest definition: the standard kilogram is a
platinum-iridium cylinder kept at he French Bureau of Weights and
measurement
8. 9. 10. 11. These are the standard SI prefixes for indicating
powers of 10. Many are familiar; yotta, zetta, exa, hecto, deka,
atto, zepto, and yocto are rarely used. 12. We will be working in
the SI system, in which the basic units are kilograms, meters, and
seconds. Quantities not in the table are derived quantities,
expressed in terms of the base units. 13. Unit conversion
- Aconversion factorsimply lets you express a quantity in terms
of other units without changing its physical value or size
- Written another way:1 = 2.54 cm/in.
- So if we have measured a length of 21.5 inches, and wish to
convert it to centimeters, we use the conversion factor:
14.
- Table shows the conversion factors between SI and British units
for length and mass only.
1 angstrom ( ) = 10 -10m 1 kg = 0.0685 slug 1 mi = 5280 ft =
1.609 km 1 lb = 0.453 592 kg 1 km = 0.621 mi 1 slug = 14.59 kg 1 in
= 2.54 cm 1 kg = 10 3g 1 m = 39.37 in = 3.281 ft Mass Length 15.
Example 1-2: The 8000-m peaks. The fourteen tallest peaks in the
world are referred to as eight-thousanders, meaning their summits
are over 8000 m above sea level. What is the elevation, in feet, of
an elevation of 8000 m? 16. Unit Analysis
- A powerful way to check your calculation is to use unit
analysis
- Not only must the numerical values on both sides of an equation
be equal, the units must be equal as well
17. Dimensions Dimensions of a quantity are the base units that
make it up; they are generally written using square brackets.
Example: Speed = distance/time Dimensions of speed: [ L / T ] In
dimensions, mass, length, time, temperature, and electric current
is symbolize with M, L, T, K and A respectively. 18.
- Dimensional analysisis the checking of dimensions of all
quantities in an equation to ensure that those which are added,
subtracted, or equated have the same dimensions.
- Example: Is this the correct equation for velocity?
Check the dimensions:Wrong!