A.2.2—Forces as interactions 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.

How it is assessed

The evidence asks why two current-carrying coils move together, rewarding an explanation based on the magnetic field produced by each coil and the resulting force on the other.

Command terms

Explain

What earns marks

Identify the two interacting bodies and use the relevant field or contact interaction to explain the force direction. For current-carrying coils, refer to the magnetic fields of the turns and state whether the resulting force is attractive or repulsive.

Watch for

Saying the coils attract because current exists, without identifying the mutual magnetic-field interaction or force direction.

Representative question

[Maximum number: 2]

Explain why, when there is a current in the coil, the separation of X and Y decreases.

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 $a_c=v^2/r=\omega^2r$ . It may come from tension, gravity, normal, friction or a field force. Angular and linear descriptions are linked by $v=\omega r=2\pi r/T$ .

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?

A.2.2—Forces as interactions between bodies

Treat Force as an Interaction Between Bodies

A.2.2 Exam Analysis

Assessment in practice

Retrieve the A.2 Forces and Momentum Model

Question 1