A.2.12—Field forces
- Syllabus
- First assessment 2025
- Objective
- —
- Level
- SL
Separate Gravitational, Electric and Magnetic Forces
Three field interactions
Gravitational, electric and magnetic forces are field forces: bodies can interact without contact. Identify the source of the field, the object acted on and the force direction.
Keep the mechanisms distinct
Gravity acts on mass, electric force acts on charge, and magnetic force acts on moving charges or currents in a magnetic field. Their equations and direction rules are not interchangeable.
Use the force relevant to the system
A free-body diagram may contain more than one field force. Add them as vectors and apply Newton’s second law to the selected body.
Common trap
Do not call every non-contact force “electromagnetic”; gravitational attraction is a separate interaction.
A.2.12 Exam Analysis
[ 1 ]
What are three fundamental forces listed in decreasing order of strength?
Retrieve the A.2 Forces and Momentum Model
Build the force model
Choose the system, draw a labelled free-body diagram, classify the interactions and resolve components. Apply Newton’s laws with the correct boundary: contact forces, field forces, friction, tension, buoyancy and restoring forces each have their own direction and conditions.
Track momentum
Use ec p=m ec v, ec J=\Delta ec p and momentum conservation only after checking external impulse. Distinguish elastic and inelastic collisions, explosions and energy transfer.
Track circular motion
The inward resultant provides ac=v2/r=ω2r. It may come from tension, gravity, normal, friction or a field force. Angular and linear descriptions are linked by v=ωr=2πr/T.
Final checks
Ask: Which body is the system? Which forces are external? Is mass constant? Is acceleration uniform or radial? Is kinetic energy conserved, transferred or increased?