SACE Physics — Stage 2
Circular Motion — 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).
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.
Last updated: March 2026 · 2 flashcards · 2 quiz questions