← Mechanics
Momentum, energy & circular motion
Momentum always conserved; KE only in elastic collisions. v = 2πr/T for circles. Exam-style 1200 kg car at 15 m s⁻¹ hits a stationary 800 kg car; they lock together Find the combined speed. Before: p = 1200 × 15 = 18,000 kg m s⁻¹ After: (1200 + 800) v = 18,000 v = 9.0 m s⁻¹ ✓ 9.0 m s⁻¹. (KE after < KE before → inelastic; energy became heat/sound.) ℹCircular-motion sentence
"The net force toward the centre (friction/tension/weight component) provides the centripetal force mv²/r." That sentence converts a description into Merit. ✕Constant SPEED is not constant VELOCITY
An object moving in a circle at a steady speed is still accelerating, because velocity is a vector and its DIRECTION keeps changing. Real papers ask this almost every year ("explain how it is possible to be moving at a constant speed yet accelerating"): the answer is that a changing direction is a changing velocity, and a changing velocity is an acceleration — even though the speed itself never changes.Naming the REAL force providing the centripetal force
- Centripetal force is never a new, separate force — it is the ROLE played by whichever real force is actually available: tension in a string, friction on a road, gravity for an orbiting satellite, or the normal force on a banked curve.
- A full-mark answer names the specific real force for the specific scenario, not just "the centripetal force" in the abstract.
- "Centrifugal force" is not a real force in this frame of reference and should never appear on a force diagram drawn from outside the rotating system.