SACE Physics — Stage 2
Circular Motion — SACE Physics — Stage 2 Flashcards & Quiz
Uniform circular motion describes an object moving in a circle at constant speed with acceleration directed toward the centre. SACE Physics Stage 2 asks you to identify the real force providing the centripetal contribution in each scenario (tension, gravity, friction, normal component) and apply F_c = mv²/r. Banked tracks and satellite orbits are standard application contexts.
Key Points
- Uniform circular motion: constant speed, changing direction; acceleration always points toward the centre (centripetal).
- Centripetal acceleration: a_c = v²/r = ω²r, where ω is angular velocity in rad/s.
- Centripetal force: F_c = mv²/r. It is a NET force label, not a new kind of force — always ask which real force(s) provide it.
- Period T = 2πr/v; angular velocity ω = 2π/T; frequency f = 1/T.
- Common real force sources: tension (string), gravity (orbits), friction (car on flat curve), component of normal (banked track).
- "Centrifugal force" is an apparent (inertial) force visible only in a rotating reference frame — it's not a real force in an inertial frame.
Common Mistakes to Avoid
- Drawing centrifugal force outward — in an inertial frame, centripetal force points INWARD and there is no outward force.
- Treating centripetal force as a separate force type — it's a label for the net inward force, which comes from a real source.
- Confusing angular velocity ω with linear velocity v — they relate by v = ωr.
- Forgetting that on a banked track, the horizontal component of the normal force provides centripetal force.
- Claiming acceleration is zero because "speed is constant" — velocity is changing direction, so acceleration is non-zero.
Exam Strategy
SACE Stage 2 circular motion questions give you a scenario and ask for centripetal force, minimum speed, or banking angle. Method: (1) draw a free-body diagram, (2) identify which real force(s) provide the centripetal contribution, (3) set that force equal to mv²/r, (4) solve for the unknown. Always state assumptions (level ground, frictionless, etc.) explicitly.
Sample Flashcards
Q1: Define uniform circular motion and derive centripetal acceleration.
Motion in a circle at constant speed. Velocity changes direction continuously, producing centripetal acceleration a_c = v²/r directed toward the centre. Also: a_c = ω²r = 4π²r/T².
Q2: What provides the centripetal force in common circular motion scenarios?
Centripetal force F_c = mv²/r is not a new force — it is the net inward force. Examples: gravity (orbits), tension (string), friction (car turning), normal force (banked track), gravitational + normal (roller coaster).
Q3: Why are some curves banked, and what provides the centripetal acceleration on flat and banked curves?
On a flat curve, the sideways friction force between the tyres and the road provides the centripetal acceleration; if friction is too small (e.g. on a wet road) the vehicle slides outwards. On a banked curve the road is tilted, so the normal force has a horizontal component directed towards the centre of the circle. This provides some or all of the centripetal acceleration, reducing the reliance on friction. At the design speed, no friction is needed: tan θ = v²/(rg).
Sample Quiz Questions
Q1: In uniform circular motion, the net force is directed toward the centre of the circle.
Answer: TRUE
The centripetal force (net inward force) maintains circular motion by continuously changing the velocity direction.
Q2: Doubling the speed in circular motion doubles the centripetal force required.
Answer: FALSE
F_c = mv²/r — centripetal force is proportional to v². Doubling speed quadruples F_c.
Revision Tip
Free-body diagrams for circular motion are visual — drill a Revizi deck with 5-6 standard scenarios (car on banked curve, satellite, pendulum, vertical loop, conical pendulum) until diagram construction is automatic.
Related Concepts
Last updated: 9 October 2026 · 3 sample flashcards · 2 sample quiz questions