Unit Overview

Forces, density and pressure

CIE A-Level Physics· 5 min read 📊 n/a

1. Unit at a Glance

We build this unit incrementally: starting with the basics of identifying individual forces, moving on to translational and rotational equilibrium, then ending with core properties of fluids. Each sub-topic builds on the previous to give you a complete toolkit for solving static mechanics problems.

Mastery of these concepts is required for almost all higher-level mechanics topics, including circular motion, gravitation, and fluid dynamics that you will cover later in the syllabus.

This unit is split into 5 connected sub-topics:

Common force types

Learn to identify, define and draw free-body diagrams for all common force types encountered in mechanics.

★⏱ 6 min

Force equilibrium

Understand and apply the conditions for translational equilibrium of rigid bodies in 1 and 2 dimensions.

★★⏱ 8 min

Moment and torque

Calculate moments of forces and use the principle of moments to solve rotational equilibrium problems.

★★★⏱ 10 min

Density

Define density and use its relationship to mass and volume to solve basic and composite object problems.

★⏱ 4 min

Fluid pressure

Derive and use the hydrostatic pressure equation for static fluids, and relate pressure to applied force.

★★⏱ 7 min

2. Common Pitfalls

Wrong move:

Forgetting that weight acts at the center of mass when calculating moments

Why:

This leads to incorrect moment arm values and wrong solutions for equilibrium problems

Correct move:

Always mark the weight force at the center of mass of an object unless explicitly stated otherwise

Wrong move:

Confusing gauge pressure and absolute pressure in hydrostatic problems

Why:

Many exam questions expect absolute pressure (including atmospheric pressure) unless stated otherwise

Correct move:

Always read the question carefully to confirm which type of pressure you are required to calculate

3. Quick Reference Cheatsheet

Concept	Key Formula/Relationship
Pressure from force	$P = \frac{F}{A}$
Density	$ρ = \frac{m}{V}$
Moment of force	$M = F \times d_{⊥}$
Translational equilibrium	Net force = $0$ in all directions
Principle of moments	Sum of clockwise moments = Sum of anticlockwise moments
Hydrostatic pressure	$P = P_{0} + ρ g h$

What's Next

Begin your study of this unit with the first sub-topic on common force types, where you will learn to correctly identify and represent forces in free-body diagrams, the starting point for all mechanics problems. Once you complete all sub-topics in this unit, you can move on to the next unit covering work, energy and power.