Chapter 1
Units
Problems: 5, 7, 16, 23, 29
Science is based on measurements and
comparisons.
When you make a measurement you are
comparing the amount to a known standard.
This leads us to systems of units:
There are two commonly used unit systems:
English (American) units
foot, pound
SI units (also called MKS or the metric system)
meters, kilogram, second
In science we use the SI system
Fundamental units:
meter – length
kilogram – mass
second – time
Ampere – electrical current
candela – luminous intensity
mole – amount
Kelvin – temperature
(m)
(kg)
(s)
(A)
(cd)
(mol)
(K)
Fundamental units are based on agreed standards.
examples:
meter: distance light travels in a vacuum over a
specific time interval
The agreed standard can be anything. As long as
people agree and are consistent.
All physical quantities can be described by
some combination of these units.
examples:
Force has units of Newtons (N) N = kg m/s2
Power has units of Watts (W)
W = kg m2/s3
When using very large are small numbers we
use scientific notation and prefixes.
1 200 000 000 m = 1.2 x 109 m = 1.2 Gm
0.000 003 s
= 3 x 10-6 s = 3 s
Some of the prefixes such as centi- (10-2) and
milli- (10-3) are common, while others such as
atto- (10-18) are only used rarely.
Changing Units
Quite often you need to change from one unit to
another. For example changing from seconds to
minutes. This is done by multiplying the initial
measurement by a conversion factor. The
conversion factor is just a ratio that equals 1.
60s
1 min
and 1min 1
1
60s
3 min =
(60 s )
(3 min)
(1 min)
180 s
In Physics 1, the units we will most often use are:
length – meter “The meter is the length of the path
traveled by light in a vacuum during a time interval
of 1/299792458 of a second.”
time – second “One second is the time taken by 9
192 631 770 oscillations of the light (of a specified
wavelength) emitted by a cesium-133 atom.”
mass – kilogram. Defined by a platinum-iridium
cylinder that is kept in France.