1. Lesson 3: SI units and prefixes
IB Physics: Chapter 1
Lesson 3: SI units and prefixes

3. Metric mishap caused loss of NASA orbiter
September 30, Web posted at: 4:21 p.m. EDT (2021 GMT).
By Robin Lloyd CNN Interactive Senior Writer
(CNN) -- NASA lost a $125 million Mars orbiter because a Lockheed Martin engineering team used English units of measurement while the agency's team used the more conventional metric system for a key spacecraft operation, according to a review finding released Thursday.
The units mismatch prevented navigation information from transferring between the Mars Climate Orbiter spacecraft team in at Lockheed Martin in Denver and the flight team at NASA's Jet Propulsion Laboratory in Pasadena, California.
Lockheed Martin helped build, develop and operate the spacecraft for NASA. Its engineers provided navigation commands for Climate Orbiter's thrusters in English units although NASA has been using the metric system predominantly since at least 1990.
Mars Reconnaissance Orbiter

4. SI Units Background
One of the most fundamental aspects of science is that it is an international endeavor. Historically people working in different countries have come upon similar discoveries, and people doing research in seemingly completely different fields of study (such as electricity and mechanics) have converged on the same phenomenon.
In order for scientists to communicate clearly across national boundaries and for scientists working in different fields of study, to be able to share ideas clearly, it helps if everyone is using similar mathematical units.
Scientists have been moving towards a common system of units since the 1800’s. But it wasn’t until 1960 that countries signed an agreement that they would work in what is now called the “SI” (Systeme International) .

5. What do you mean “in which any physical measurement can be written?
SI Base Units
The International System (SI) of units has 7 “Base Units” in which which any physical quantity can be written. These Base Units are:
What do you mean “in which any physical measurement can be written?
Measurement
Unit
Symbol
Time
Length
Mass
Electric Current
Temperature
Intensity of Light
Amount of Matter
Seconds s
Metres m
Kilograms Kg
Ampere A
Kelvin K
Candela cd
Mole
Mol

6. Other Measurements?
Ok, so distance is measured in “metres”, time is measured in seconds and mass is measured in kilograms. We can now discuss other physical quantities in terms of these units. For example.
Acceleration is measured in “metres per second squared” (ms-2)
Force is measured in “kilograms meters per second squared” (kgms-2)
Energy is measured in “kilograms metres squared per second squared” (kgm2s-2)

7. Derived Units
Some common units are really annoying to say (eg kilograms metres squared per second squared) so instead we give a number of “derived units” their own name. Each of these units represent their own measurements, but actually are a combination of the other seven base units.
Examples are:
Newtons (kgms-2)
Joules (kgm2s-2)
Ohm (m2kgs-3A-2)

8. Conversion From Base SI Units
Derived Units
Measurement
Unit
Symbol
Conversion From Base SI Units
radioactivity
s-1
electric charge
A s
frequency
energy or work
kg m2 s-2
force or weight
kg m s-1
resistance
kg m2 s-3 A-2
pressure
kg m-1 s-2
magnetic field strength
kg s-2 A-1
potential difference
kg m2 s-3 A-1
power
kg m2 s-3
magnetic flux
kg m2 s-2 A-1
Becquerel Bq
Coulomb C
Hertz Hz
Joules J
Newton N
Ohm Ω
Pascal Pa
Tesla T
Volt V
Watt W
Webber Wb

9. Conversion to Base SI Units
Other common units
Name
Symbol
Measurement
Conversion to Base SI Units
litre l
volume
1 litre = 1x10-3 m3
minute
min
time
1 min = 60 s
hour hr
1 hour = 3600 s
day
1 day = s
year yr
1 yr = x107 s
kilowatt-hour
kWh
energy
1 kWh = 3.6x106 J
ElectronVolt eV
1 eV = x10-19
degrees Celsius ºC
temperature
0ºC = 273K
unified atomic mass unit u
mass (of nucleon)
1 u = x10-27 kg
These units are commonly used in science, although they are not officially “SI Units”.

10. Prefixes
We also add Prefixes to units to indicate an order of magnitude.
Eg 7 km the “k” is a prefix on the meter and indicates “kilo” which means 1000 (or 103)
So 7km is 7x103 metres or 7000 m

11. Prefixes Prefix Symbol Factor tera giga mega kilo - centi milli micro
nano
pico
femto T
1012 G
109 M
106 k
103 T
10-2 T
10-3 T
10-6 T
10-9 T
10-12 T
10-15

12. References
Lloyd, R “Metric mishap caused loss of NASA orbiter” CNN. accessed June 26, 2015.
Waste, C. "Mars Reconnaissance Orbiter, front view, artist's concept (PIA07245)" by NASA/JPL/Corby Waste - (image link). Licensed under Public Domain via Wikimedia Commons - ,_front_view,_artist%27s_concept_(PIA07245).jpg#/media/File:Mars_Rec onnaissance_Orbiter,_front_view,_artist%27s_concept_(PIA07245).jpg