Unit Overview
Forces, density and pressure
CIE A-Level PhysicsΒ· 5 min read π 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.
β β± 6 min
Force equilibrium
Understand and apply the conditions for translational equilibrium of rigid bodies in 1 and 2 dimensions.
β β β± 8 min
Moment and torque
Calculate moments of forces and use the principle of moments to solve rotational equilibrium problems.
β β β β± 10 min
Density
Define density and use its relationship to mass and volume to solve basic and composite object problems.
β β± 4 min
Fluid pressure
Derive and use the hydrostatic pressure equation for static fluids, and relate pressure to applied force.
β β β± 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 | |
Density | |
Moment of force | |
Translational equilibrium | Net force = in all directions |
Principle of moments | Sum of clockwise moments = Sum of anticlockwise moments |
Hydrostatic pressure |
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.