# Kinematics

> CIE A-Level Physics · CIE 9702 (2022-2024+)
> Source: https://www.owlsprep.com/study/cie-9702-u2-overview/
> Weight: n/a

This unit introduces core concepts that describe motion of objects, forming the foundation for all further study of mechanics in A-Level Physics.

**Prerequisites:** [Basic physical quantities and SI units from Unit 1](https://www.owlsprep.com/study/cie-9702-u1-overview/)

## Learning objectives

- Distinguish between scalar and vector kinematic quantities used to describe motion
- Interpret and construct displacement-time and velocity-time graphs for moving objects

## Unit at a glance

Kinematics is the study of how objects move, without yet considering the forces that cause motion. We start by defining key quantities, then build up to solving problems for straight-line motion and curved projectile motion.

This learning path progresses from foundational definitions to practical problem solving, with graphical methods building intuition before we introduce algebraic equations for accelerated motion.

This unit is split into the following sub-topics:
- [Distance, displacement, speed, velocity and acceleration](https://www.owlsprep.com/study/cie-9702-u2-distance-displacement-speed-velocity-and/) — Distinguishes scalar vs vector quantities and defines core kinematic terms for motion.
- [Graphical representation of motion](https://www.owlsprep.com/study/cie-9702-u2-graphical-representation-of-motion/) — Teaches how to interpret and draw displacement-time and velocity-time graphs for motion.
- [Uniformly accelerated motion equations](https://www.owlsprep.com/study/cie-9702-u2-uniformly-accelerated-motion-equations/) — Derives and applies the 4 suvat equations for motion with constant acceleration.
- [Free fall](https://www.owlsprep.com/study/cie-9702-u2-free-fall/) — Analyzes motion of objects accelerating under gravity only, including experimental methods.
- [Projectile motion](https://www.owlsprep.com/study/cie-9702-u2-projectile-motion/) — Resolves projectile motion into horizontal and vertical components to solve trajectory problems.

## Common pitfalls

- **Wrong:** Mixing up scalar (distance/speed) and vector (displacement/velocity) quantities in calculations
  - Why it fails: This leads to incorrect signs and wrong final answers, especially for motion that changes direction
  - Correct: Always confirm whether a question asks for scalar or vector values, and assign signs correctly based on your reference direction
- **Wrong:** Using suvat equations for motion with non-uniform acceleration
  - Why it fails: The suvat equations are only derived for constant acceleration, so they give incorrect results for changing acceleration
  - Correct: Use graphical methods to solve problems involving non-uniform acceleration in this syllabus
- **Wrong:** Treating horizontal acceleration as non-zero for standard projectile motion
  - Why it fails: Ignoring the standard assumption of negligible air resistance leads to incorrect trajectory calculations
  - Correct: Always assume $a_x = 0$ for horizontal projectile motion, unless the question explicitly states otherwise

## Cheatsheet

| Concept | Key Formula/Rule |
| --- | --- |
| Average speed | $v_{av} = \frac{\text{total distance}}{\text{total time}}$ |
| Gradient of displacement-time graph | Equal to velocity |
| Area under velocity-time graph | Equal to displacement |
| Gradient of velocity-time graph | Equal to acceleration |
| SUVAT equations (constant $a$) | $v = u + at$, $s = ut + \frac{1}{2}at^2$, $v^2 = u^2 + 2as$, $s = \frac{(u+v)}{2}t$ |
| Free fall acceleration | $a = g = 9.81 \text{ m s}^{-2}$ downwards |
| Projectile horizontal motion | $a_x = 0$, $v_x = u_x = \text{constant}$ |
| Projectile vertical motion | $a_y = g$, follows uniform acceleration rules |

## What's next

Begin your study of this kinematics unit with the first sub-topic, which introduces the core scalar and vector definitions you will rely on for all subsequent topics in mechanics. After completing all sub-topics in this unit, you will progress to Unit 3: Dynamics, which builds on kinematic concepts to explain how forces cause changes in motion.

- [Distance, displacement, speed, velocity and acceleration](https://www.owlsprep.com/study/cie-9702-u2-distance-displacement-speed-velocity-and/)
- [Unit 3: Dynamics Overview](https://www.owlsprep.com/study/cie-9702-u3-overview/)
- [Graphical representation of motion](https://www.owlsprep.com/study/cie-9702-u2-graphical-representation-of-motion/)

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