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  • Perform unit conversions both in the SI and English units.
  • Explain the most common prefixes in the SI units and be able to write them in scientific notation.
A view of Earth from the Moon.
The distance from Earth to the Moon may seem immense, but it is just a tiny fraction of the distances from Earth to other celestial bodies. (credit: NASA)

The range of objects and phenomena studied in physics is immense. From the incredibly short lifetime of a nucleus to the age of the Earth, from the tiny sizes of sub-nuclear particles to the vast distance to the edges of the known universe, from the force exerted by a jumping flea to the force between Earth and the Sun, there are enough factors of 10 to challenge the imagination of even the most experienced scientist. Giving numerical values for physical quantities and equations for physical principles allows us to understand nature much more deeply than does qualitative description alone. To comprehend these vast ranges, we must also have accepted units in which to express them. And we shall find that (even in the potentially mundane discussion of meters, kilograms, and seconds) a profound simplicity of nature appears—all physical quantities can be expressed as combinations of only four fundamental physical quantities: length, mass, time, and electric current.

We define a physical quantity    either by specifying how it is measured or by stating how it is calculated from other measurements. For example, we define distance and time by specifying methods for measuring them, whereas we define average speed by stating that it is calculated as distance traveled divided by time of travel.

Measurements of physical quantities are expressed in terms of units    , which are standardized values. For example, the length of a race, which is a physical quantity, can be expressed in units of meters (for sprinters) or kilometers (for distance runners). Without standardized units, it would be extremely difficult for scientists to express and compare measured values in a meaningful way. (See [link] .)

A boy looking at a map and trying to guess distances with unit of length mentioned as cables between two points.
Distances given in unknown units are maddeningly useless.

There are two major systems of units used in the world: SI units    (also known as the metric system) and English units    (also known as the customary or imperial system). English units were historically used in nations once ruled by the British Empire and are still widely used in the United States. Virtually every other country in the world now uses SI units as the standard; the metric system is also the standard system agreed upon by scientists and mathematicians. The acronym “SI” is derived from the French Système International .

Si units: fundamental and derived units

[link] gives the fundamental SI units that are used throughout this textbook. This text uses non-SI units in a few applications where they are in very common use, such as the measurement of blood pressure in millimeters of mercury (mm Hg). Whenever non-SI units are discussed, they will be tied to SI units through conversions.

Fundamental si units
Length Mass Time Electric Current
meter (m) kilogram (kg) second (s) ampere (A)

Questions & Answers

advantages of CRO over ordinary voltmeter
Dismas Reply
what is the difference between displacement and distance?!
Daniel Reply
what is equilibrium
Sade Reply
If a system is said to be under equilibrium whenever there is no force act upon it... And it remain in its initial stage..
What is conductivity
Saud Reply
It is the ease with which electrical charges or heat can be transmitted through a material or a solution.
how to find magnitude and direction
Arjune Reply
how to caclculate for speed
derivation of ohms law
Kazeem Reply
derivation of resistance
R=v/I where R=resistor, v=voltage, I=current
A puck is moving on an air hockey table. Relative to an x, y coordinate system at time t 0 s, the x components of the puck’s ini￾tial velocity and acceleration are v0x 1.0 m/s and ax 2.0 m/s2 . The y components of the puck’s initial velocity and acceleration are v0y 2.0 m/s and ay 2.0
Electric current is the flow of electrons
Kelly Reply
is there really flow of electrons exist?
Yes It exists
explain plz how electrons flow
if electron flows from where first come and end the first one
an electron will flow accross a conductor because or when it posseses kinectic energy
electron can not flow jist trasmit electrical energy
free electrons of conductor
electric means the flow heat current.
Serah Reply
electric means the flow of heat current in a circuit.
What is electric
Manasseh Reply
electric means?
electric means the flow of heat current in a circuit.
electric means the flow of electric current through conductor
the continuos flow of electrons in a circuit is called electric
electric means charge
electric means current
a boy cycles continuously through a distance of 1.0km in 5minutes. calculate his average speed in ms-1(meter per second). how do I solve this
Jenny Reply
speed = distance/time be sure to convert the km to m and minutes to seconds check my utube video "mathwithmrv speed"
d=1.0km÷1000=0.001 t=5×60=300s s=d\t s=0.001/300=0.0000033m\s
A puck is moving on an air hockey table. Relative to an x, y coordinate system at time t 0 s, the x components of the puck’s ini￾tial velocity and acceleration are v0x 1.0 m/s and ax 2.0 m/s2 . The y components of the puck’s initial velocity and acceleration are v0y 2.0 m/s and ay 2.0
D=1km=1000m t=5mins×60secs=300sec s=d/t=3.333m/s
I think Daniel Glorious is ryt
why we cannot use DC instead of AC in a transformer
kusshaf Reply
becuse the d .c cannot travel for long distance trnsmission
what is physics
Chiwetalu Reply
branch of science which deals with matter energy and their relationship between them
Life science
what is heat and temperature
Kazeem Reply
how does sound affect temperature
Clement Reply
sound is directly proportional to the temperature.
how to solve wave question
Wisdom Reply
I would like to know how I am not at all smart when it comes to math. please explain so I can understand. sincerly
Just know d relationship btw 1)wave length 2)frequency and velocity
First of all, you are smart and you will get it👍🏽... v = f × wavelength see my youtube channel: "mathwithmrv" if you want to know how to rearrange equations using the balance method
nice self promotion though xD
thanks dear
hi pls help me with this question A ball is projected vertically upwards from the top of a tower 60m high with a velocity of 30ms1.what is the maximum height above the ground level?how long does it take to reach the ground level?
what is scalar quantities
scaler quantity are quanties that have only direction and no magnitude
ice Point
scalar quantity are quantities that have magnitude but no direction

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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