Fields & Gravitation is a core part of IB Physics. Revise the key concepts and common mistakes below, then lock them in with the free games.
Key concepts
Period TTime for 1 revolution
Frequency f1/T (Hz)
Angular velocity ω2π/T (rad/s)
Centripetal av²/r = ω²r (toward centre)
Centripetal Fmv²/r
Tangential vLinear speed = rω
Gravitational potential V-GM/r (J/kg)
Potential energymV = -GMm/r
ReferenceV = 0 at infinity
Total orbital E½mv² + PE = -GMm/(2r)
Escape velocityv = √(2GM/r)
EquipotentialSurface of constant V
GravitationF = GMm/r²
G6.67 × 10⁻¹¹ N m²/kg²
Common mistakes to avoid
Questions where students often pick the tempting wrong answer — make sure you know the right one:
A car moves at a steady speed in a straight line. What is the resultant force?✗ A forward force, because it keeps moving. ✓ Zero — the forces are balanced at constant velocity.
How do mass and weight differ?✗ They are the same quantity in different units. ✓ Mass (kg) is the amount of matter; weight (N) is the gravitational force on it.
Can gravitational fields repel objects?✗ Yes, gravity can push objects apart. ✓ No — gravity is always attractive; electric/magnetic fields can attract or repel.
How does a radial field's strength change with distance?✗ It is the same at all distances. ✓ It follows an inverse-square law, weakening with distance.
Can field lines cross one another?✗ Yes, field lines cross where two fields meet. ✓ No — crossing would mean two field directions at one point.
Practise Fields & Gravitation — free games
Test yourself with these quick revision games for this topic:
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