Cambridge A-Levels A* Physics Notes
Mr.Gabriel
1.1 Physical Quantities
All physical quantities consist of a numerical magnitude and a unit and can be expressed in
terms of the SI base units:
The base units are:
•
•
•
mass (Kg)
length (m)
time (s)
electric current (A)
temperature (K)
amount of substance (mol)
Derived units:
All units can be derived from products of the base units – it’s just simple algebra:
Eg:
Force in Newtons – express this in base units:
F= ma = kg x m/s2
Newton = kilogram metre per second squared
Often shown as: kgm/s2 OR kgms-2
Q1: Derive the following into correct base units:
a) Speed
b) Acceleration
c) Pressure
d) Kinetic Energy
e) Potential Energy
f) Voltage
g) Resistance
h) Friction
1 . 1 Physical Quantities & 1 . 2 SI units
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Cambridge A-Levels A* Physics Notes
Mr.Gabriel
1.2 SI Units
Homogeneity of an equation
An equation is homogeneous if quantities on BOTH sides of the equation has the same unit.
Example
s = ut + ½ at2
•LHS : unit of s = m
•RHS : unit of ut = ms-1 x s = m
• unit of at2 = ms-2 x s2 = m
•Unit on LHS = unit on RHS
•Hence equation is homogeneous
P = ρgh2
LHS ; unit of P = Nm-2 = kgm-1s-2
RHS : ρgh2 = kgm-3(ms-2)(m2) = kgs-2
•Unit on LHS = unit on RHS
•Hence equation is not homogeneous
Q2 : Prove by deriving into correct base units that the following equations are
dimensionally correct:
a) Energy = mass x speed of light squared
b) Acceleration due to gravity = Weight
Mass
c) Force = mass x acceleration
d) Work = force x distance
e) Power = Energy / Time
f) Charge = Work done / Volts
1 . 1 Physical Quantities & 1 . 2 SI units
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Cambridge A-Levels A* Physics Notes
Mr.Gabriel
Prefixes
•For very large or very small numbers, we can use standard prefixes with the base units.
•The main prefixes that you need to know are
shown in the table.
Prefix
Tera
Abbreviation
T
Power
10^12
Giga
Mega
Kilo
Hecto
G
M
K
H
10^9
10^6
10^3
10^2
Deka
Deci
Centi
D
d
c
10^1
10^-1
10^-2
Mili
Micro
Nano
m
µ
n
10^-3
10^-6
10^-9
Pico
p
10^-12
The Great Mighty King Henry died drinking chocolate milk until nine pm
Questions
Q3: Write the following out in full:
Eg: 1 km = 1 x 103m = 1000metres
a) 10kJ
b) 2.56kV
10kJ= 10x10^3J = 10000J
c) 10cm
d) 100 mg
e) 5 MJ
f) 1.234 TW
g) 4.56 nK
h) 12 pHz
Q4: convert the following to the nearest power of ten symbol:
a) 10000m
b) 0.00004 A
c) 12000000C
d) 0.0000000612mol
1 . 1 Physical Quantities & 1 . 2 SI units
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Cambridge A-Levels A* Physics Notes
Mr.Gabriel
Q5 : Converting Prefixes
1. The mass of Earth is 5.98 × 1024 kg. What is the mass expressed in megagrams?
2. Light travels through the universe at a velocity of 3.00 × 1010 cm/s. How many gigameters
does light travel in one second?
Answer: 0.300 Gm (3.00 × 10–1 Gm)
3. Convert 1m2 to cm2
4. Convert 100km/h to m/s
5. Convert 9.8kgm2/s2 to gcm2/s2
Labelling graph axes and table columns
When labelling graph and table
1st Write symbol for physical quantity
2nd Write the "/"
For example when making table for
Acceleration, Time graph
a/ms-2
t/s
3rd Write unit
1 . 1 Physical Quantities & 1 . 2 SI units
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Cambridge A-Levels A* Physics Notes
1 . 1 Physical Quantities & 1 . 2 SI units
Mr.Gabriel
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Cambridge A-Levels A* Physics Notes
1 . 1 Physical Quantities & 1 . 2 SI units
Mr.Gabriel
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Cambridge A-Levels A* Physics Notes
1 . 1 Physical Quantities & 1 . 2 SI units
Mr.Gabriel
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Cambridge A-Levels A* Physics Notes
1 . 1 Physical Quantities & 1 . 2 SI units
Mr.Gabriel
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Cambridge A-Levels A* Physics Notes
1 . 1 Physical Quantities & 1 . 2 SI units
Mr.Gabriel
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Cambridge A-Levels A* Physics Notes
1 . 1 Physical Quantities & 1 . 2 SI units
Mr.Gabriel
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Cambridge A-Levels A* Physics Notes
1 . 1 Physical Quantities & 1 . 2 SI units
Mr.Gabriel
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