Edexcel IAL Physics revision · A2 — Mechanics & fields
Circular motion
The first A2 topic and the foundation for everything with 'fields' in the name. One idea does all the work: circular motion at constant speed is still acceleration, because the direction of the velocity is always changing.
Dr Desouky Physics Academy · Meet your physics tutor
What the syllabus demands
- —Define the radian; convert between degrees and radians
- —Define angular velocity ω; relate it to period and speed: v = rω
- —State that centripetal acceleration a = v²/r = rω² is directed towards the centre
- —Identify the physical force providing the centripetal force in each situation
- —Analyse motion in horizontal and vertical circles
Definitions that earn marks
Clear definitions to practise — check your course mark scheme
- Radian
- The angle subtended at the centre of a circle by an arc equal in length to the radius. 2π radians = 360°.
- Angular velocity (ω)
- The angle swept out per unit time: ω = 2π ÷ T. Units: rad/s.
- Centripetal force
- The resultant force acting towards the centre of the circle, required to keep a body moving in circular motion. It is not a new force — it is provided by tension, gravity, friction or a normal force.
The equations
More equations to practise: the Edexcel IAL formula sheet.
Where the marks die
Common mistakes to check
- 01
Adding a 'centripetal force' to the free-body diagram. Centripetal force is the resultant of the real forces — drawing it as an extra force double-counts and wrecks the algebra.
- 02
Saying the speed changes in uniform circular motion. Speed is constant; velocity changes because direction changes. The acceleration is perpendicular to the velocity, so it never speeds the object up.
- 03
At the top of a vertical circle, forgetting both weight and tension (or normal force) point towards the centre: mg + T = mv²/r. The minimum-speed condition is T = 0, giving v² = gr.
- 04
Degrees left in the calculator. Angular equations assume radians — one mode error can void a whole question.
One worked example, done properly
Question
A 0.30 kg ball on a string is whirled in a horizontal circle of radius 0.80 m at 2.0 revolutions per second. Find the tension in the string.
Method
- 1.ω = 2π × 2.0 = 12.6 rad/s.
- 2.F = mrω² = 0.30 × 0.80 × (12.6)² = 0.30 × 0.80 × 158.
T ≈ 38 N