HSC Physics — Module 1
Projectile Motion — Flashcards & Quiz
Projectile motion is a key topic in HSC Physics Module 1, where an object moves under gravity with no other forces acting (ignoring air resistance). The fundamental principle is that horizontal and vertical motions are independent — horizontal velocity stays constant while vertical velocity changes at 9.8 m/s². You need to resolve initial velocities into components, calculate time of flight, range, and maximum height. Exam questions frequently require multi-step calculations combining kinematics equations with vector decomposition.
Key Points
- Projectile motion combines two independent motions: constant horizontal velocity and constant vertical acceleration (g = 9.8 m s⁻² downward).
- Split initial velocity into components: u_x = u cos θ, u_y = u sin θ. Treat horizontal and vertical separately.
- Time of flight from u_y = -u_y at the same height: t = 2 u sin θ / g (no air resistance, level ground).
- Range R = u² sin(2θ) / g; max range at θ = 45° (no air resistance).
- Max height H = (u sin θ)² / (2g); velocity at top is horizontal only (v_y = 0).
- Common trap: air resistance is NOT zero in practice — but HSC problems usually assume it is; always state assumptions.
Common Mistakes to Avoid
- Mixing horizontal and vertical motion — they are independent and should be analysed separately.
- Forgetting that horizontal velocity is constant (no air resistance) — only vertical velocity changes under gravity.
- Using sin θ for horizontal component — it's u cos θ horizontal, u sin θ vertical when θ is measured from horizontal.
- Forgetting signs when up is positive — g is negative, so deceleration on the way up and acceleration on the way down both come out of the same g = -9.8.
- Assuming maximum range is always at 45° — it's only true on level ground with no air resistance.
Exam Strategy
HSC Module 1 projectile motion questions give an initial velocity and angle, then ask for time of flight, range, maximum height or velocity at a given time. Method: (1) resolve the initial velocity into horizontal (u cos θ) and vertical (u sin θ) components, (2) treat the two directions independently using SUVAT equations, (3) use the vertical equation to find time, then plug into horizontal for range. Always state assumptions (e.g. "neglecting air resistance").
Sample Flashcards
Q1: Describe projectile motion and its two components.
Projectile motion is the motion of an object launched into the air under gravity only. It has two independent components: horizontal (constant velocity, a_x = 0) and vertical (constant acceleration, a_y = g = 9.8 m/s² downward). The path is a parabola.
Q2: How do you calculate the range and maximum height of a projectile?
Time of flight: t = 2u sinθ/g. Maximum height: H = u²sin²θ/(2g). Range: R = u²sin2θ/g. Maximum range occurs at θ = 45°. These formulas assume launch and landing are at the same height.
Q3: Why does air resistance affect projectile motion?
Air resistance (drag) opposes motion in both horizontal and vertical directions. Horizontally, it decelerates the projectile (v_x is no longer constant). Vertically, it reduces both the maximum height and time of flight. The trajectory becomes asymmetric — steeper on descent.
Q4: What happens at the peak of a projectile's trajectory?
At the peak: vertical velocity v_y = 0 (momentarily stationary in the vertical direction). Horizontal velocity v_x remains unchanged (no horizontal acceleration). The object is still moving horizontally. Acceleration = g = 9.8 m/s² downward (never zero during flight).
Sample Quiz Questions
Q1: The horizontal velocity of a projectile remains constant throughout its flight (ignoring air resistance).
Answer: TRUE
With no air resistance, there is no horizontal force, so horizontal acceleration = 0. By Newton's first law, horizontal velocity remains constant throughout the flight.
Q2: The acceleration of a projectile is zero at the highest point of its trajectory.
Answer: FALSE
Acceleration due to gravity is CONSTANT at 9.8 m/s² downward throughout the entire flight, including at the peak. Only the vertical VELOCITY is zero at the highest point, not the acceleration.
Q3: A ball thrown horizontally from a cliff takes longer to reach the ground than one dropped vertically from the same height.
Answer: FALSE
Both take the same time. Horizontal and vertical motions are independent. The horizontal velocity doesn't affect the vertical fall time, which depends only on height and g.
Revision Tip
Projectile motion is a pattern-based topic — drill a Revizi flashcard deck with 8+ scenarios (horizontal launch, angled launch, from a cliff, landing at different height) until the decomposition is automatic.
Related Concepts
Last updated: March 2026 · 4 flashcards · 4 quiz questions