# Dynamics

> CIE A-Level Physics · 9702
> Source: https://www.owlsprep.com/study/cie-9702-u3-overview/
> Weight: n/a

This unit introduces core concepts of force, motion, and momentum that form the foundation of Newtonian mechanics, essential for all further topics in A-Level Physics.

**Prerequisites:** [Kinematics (Unit 2 CIE 9702)](https://www.owlsprep.com/study/cie-9702-u2-overview/)

## Learning objectives

- Understand and apply Newton's three laws of motion to a range of classical mechanics problems
- Define linear momentum and use the principle of conservation of momentum to solve collision and explosion problems
- Distinguish between the physical properties of mass and weight, and calculate weight in different gravitational fields
- Classify collisions as elastic or inelastic and apply relevant conservation rules to each type

## Unit at a Glance

We build this unit from foundational definitions up to powerful problem-solving tools for interacting objects. We start with core definitions of mass and weight, then move to Newton's laws of motion that describe how forces change motion. From Newton's second law, we derive the concept of linear momentum, then extend this to the conservation of momentum, which we apply to all types of collisions.

This unit is split into 5 connected sub-topics:
- [Mass and weight](https://www.owlsprep.com/study/cie-9702-u3-mass-and-weight/) — Distinguish between mass and weight, and calculate weight for any given gravitational field strength.
- [Newton's laws of motion](https://www.owlsprep.com/study/cie-9702-u3-newton-s-laws-of-motion/) — Learn each of Newton's three laws and apply them to solve problems involving force and motion.
- [Linear momentum](https://www.owlsprep.com/study/cie-9702-u3-linear-momentum/) — Define linear momentum and relate it to force via the general form of Newton's second law.
- [Conservation of momentum](https://www.owlsprep.com/study/cie-9702-u3-conservation-of-momentum/) — Derive and apply the principle of conservation of momentum for isolated systems of interacting objects.
- [Collisions](https://www.owlsprep.com/study/cie-9702-u3-collisions/) — Classify collisions as elastic or inelastic and solve problems for impacts and explosions.

## Common pitfalls

- **Wrong:** Confusing mass and weight
  - Why it fails: Mass is an invariant property of a body, while weight is a force that changes with gravitational field strength.
  - Correct: Always remember $W = mg$: weight is a force measured in newtons, mass is measured in kilograms.
- **Wrong:** Applying conservation of momentum to non-isolated systems
  - Why it fails: Momentum is only conserved when no net external force acts on the system of interacting objects.
  - Correct: Check for external forces like friction or gravity before applying the conservation principle.
- **Wrong:** Assuming kinetic energy is conserved in all collisions
  - Why it fails: Only perfectly elastic collisions conserve kinetic energy; most everyday collisions are inelastic.
  - Correct: Only use kinetic energy conservation when you confirm the collision is explicitly elastic.

## Cheatsheet

| Concept / Formula | Description |
| --- | --- |
| $W = mg$ | Weight $W$ of mass $m$ in gravitational field $g$ |
| $F = ma$ | Newton's second law for constant mass $m$ |
| $p = mv$ | Linear momentum $p$ of mass $m$ moving at velocity $v$ |
| $F = \frac{\Delta p}{\Delta t}$ | General form of Newton's second law |
| $\sum p_{\text{initial}} = \sum p_{\text{final}}$ | Conservation of momentum for isolated systems |
| $\Delta KE = 0$ | Defining property of a perfectly elastic collision |

## What's next

Start your learning of this unit with the first foundational sub-topic, Mass and Weight, which introduces core definitions you will use for all subsequent topics on force and momentum in this unit. Once you complete all sub-topics in Dynamics, you can move on to the next unit in the CIE 9702 syllabus.

- [Mass and weight](https://www.owlsprep.com/study/cie-9702-u3-mass-and-weight/)
- [Forces, Density and Pressure (Unit Overview)](https://www.owlsprep.com/study/cie-9702-u4-overview/)
- [Newton's laws of motion](https://www.owlsprep.com/study/cie-9702-u3-newton-s-laws-of-motion/)

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From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/cie-9702-u3-overview/