1. Gravitational Forces
What is gravity?
Gravity is a force of attraction between objects that have mass.
The bigger the mass → the stronger the gravity.
The closer the objects → the stronger the gravity.
Mass vs Weight
Mass
Amount of matter in an object
Measured in kilograms (kg)
Stays the same everywhere
Measured with a balance
Weight
Force of gravity acting on an object
Measured in newtons (N)
Changes depending on gravity
Measured with a spring scale
Example
Your mass = 50 kg (same on Earth and Moon)
Your weight = different (because gravity changes)
Formula for Weight
F = mg
F = weight (N)
m = mass (kg)
g = gravitational field strength
Earth: 9.8 N/kg (≈ 10 N/kg)
Example
A boy has a mass of 40 kg. What is his weight?
F = mg
F = 40 × 10
F = 400 N
2. Unit Conversions
1 kg = 1000 g
1 g = 0.001 kg
1 Mg = 1 000 000 g
1 g = 0.000001 Mg
Tips
kg → g = ×1000
g → kg = ÷1000
g → Mg = ÷1 000 000
Mg → g = ×1 000 000
3. Magnetism
Magnetic vs Non-Magnetic Materials
Magnetic: iron, steel, nickel, cobalt
Non-magnetic: plastic, wood, glass
Uses of Magnets
Fridges
Compasses
Electric motors
Speakers
Magnetic Poles
North pole (N)
South pole (S)
Rule
Like poles repel (N–N, S–S)
Unlike poles attract (N–S)
Magnetic Fields
Area around a magnet where force is felt
Represented by field lines
Field Line Rules
Go from North → South
Never cross
Closer lines = stronger field
4. Electrostatics
What is electrostatics?
Study of charges at rest
Types of Charges
Positive (+)
Negative (−)
Rule
Like charges repel
Unlike charges attract
Charging by Friction
When two objects are rubbed together
Electrons move from one object to another
One becomes negative, the other positive
Example
Rub a balloon on your hair → it sticks to the wall
Interaction of Charges
Positive + Positive → repel
Negative + Negative → repel
Positive + Negative → attract
5. Polarisation (NEW)
What is polarisation?
Polarisation happens when charges in a neutral object rearrange due to a nearby charged object
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