UNITS AND MEASUREMENTS
Fundamental Quantity
Basic physical quantity
Basic unit Symbol
Length
metre
m
Mass
kilogram
k
Time
second
s
Electric current
Ampere
A
UNITS AND MEASUREMENTS
Absolute temperature
Kelvin
K
Amount of matter
Mole
mol
Luminous intensity
Candela
cd
Plane angle
radian
rad
Solid angle
steradian
sr
Supplementary Quantities
UNITS AND MEASUREMENTS
Length :
1 ly = 9.4673 x 1015m
1 AU = 1.496 x 1011 m
1 parsec = 3.08 x 1016 m
1 par sec > 1 ly > 1 AU
UNITS AND MEASUREMENTS
• To convert a unit of given physical quantities from one system to another system for which
a
b
c
 M1   L1   T1 
n
=
n
we use 2 1 
     .
M
 2   L2   T 2 
• Error = True value - Measured value
UNITS AND MEASUREMENTS
Let a physical quantity be measured n times. Let the measured
values be a1, a2, a3, .., an.
• Absolute error = am = a1 + a2 + ..... + an
n
• Mean absolute error =
• Relative error = δa =
∆amean =
∆amean
am
a1 + a2 + ..... + an
n
UNITS AND MEASUREMENTS
∆amean
× 100%
• Percentage error =
am
• Error in Power : Let Z =
xa
yb
% error in Z = a( % error in x) + b(% error in y)
UNITS AND MEASUREMENTS
Combination of Errors:
• If ∆a and ∆b are the errors in ‘a & b; the errors in
a ± b = ± ( ∆a + ∆b ) .
• If Z = AmBnCl etc., then maximum fractional error in Z is
given by
∆Z
∆A
∆B
∆C
=m
+n
+l
Z
A
B
C