A.2.2—Forces as interactions between bodies
- Syllabus
- First assessment 2025
- Objective
- —
- Level
- SL
Treat Force as an Interaction Between Bodies
A force needs an interaction
A force is an interaction between bodies. One body exerts the force and another body experiences it. Contact, gravitational, electric and magnetic interactions can all change momentum.
Name both bodies
When explaining a force, state the interacting pair and the direction of the force on the chosen body. The reaction force acts on the other body, not back on the same free-body diagram.
Fields can mediate interaction
Bodies do not need to touch for gravitational, electric or magnetic forces. For example, current-carrying coils interact through their magnetic fields, producing attraction or repulsion depending on the field arrangement.
Common trap
Do not describe a force as a property that an isolated object “has” without naming the other body or field involved.
A.2.2 Exam Analysis
[ 2 ]
Explain why, when there is a current in the coil, the separation of X and Y decreases.
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?