A.2.1—Newton’s three laws of motion

Three linked laws

If the resultant force is zero, velocity is constant.
A resultant force changes momentum; for constant mass, $\vec F_{net}=m\vec a$ .
Forces between two bodies are equal in magnitude and opposite in direction, acting on different bodies.

Choose the system

Draw forces acting on the chosen object, then use the resultant force to predict its acceleration. For action–reaction pairs, identify the two different bodies before applying the third law.

Use interactions to explain motion

A rocket pushes gas backward; the gas exerts an equal and opposite force on the rocket. The rocket can therefore accelerate even in the absence of a supporting surface.

Common trap

The forces in a third-law pair do not cancel in one free-body diagram because they act on different objects.

How it is assessed

The evidence tests an engine slowing a probe using Newton’s second/third law reasoning and asks for the direction of the net force on a projectile.

Command terms

Explain / Identify

What earns marks

Name the chosen object and the resultant force. For a rocket, explain the force pair or momentum transfer to expelled gas and connect the resulting force to deceleration or acceleration. For a projectile, use the net force direction, not the velocity direction.

Watch for

Treating the equal and opposite third-law forces as acting on the same object or confusing velocity direction with net-force direction.

Representative question

[Maximum number: 3]

As the probe approaches the surface of the asteroid, a rocket engine is fired to slow its descent. Explain how the engine changes the speed of the probe.

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.2.1—Newton’s three laws of motion

Use Newton’s Three Laws

A.2.1 Exam Analysis

Assessment in practice

Retrieve the A.2 Forces and Momentum Model

Question 1