WACE Physics · Unit 3
WACE Physics Unit 3: Gravity & Motion — Flashcards & Quiz
WACE Physics ATAR Unit 3 covers gravity and motion — from Newton's law of universal gravitation to orbital mechanics. These free flashcards and true/false questions help you revise gravitational field strength, projectile motion, uniform circular motion, centripetal force, Kepler's laws, satellite motion, gravitational potential energy and escape velocity. Every card is aligned to the SCSA syllabus so you study exactly what appears in your WACE ATAR exams.
Key Terms
- Gravitational Field Strength
- The gravitational force per unit mass experienced by a small test mass at a point in a gravitational field, measured in N/kg and calculated as g = GM/r squared. The SCSA WACE Physics ATAR Unit 3 course requires students to calculate and interpret field strength at various distances from a mass.
- Newton's Law of Universal Gravitation
- Every mass attracts every other mass with a force proportional to the product of their masses and inversely proportional to the square of the distance between their centres (F = GMm/r squared). SCSA expects WACE ATAR students to apply this law to planetary, satellite and everyday scenarios.
- Orbital Velocity
- The speed at which an object must travel to maintain a stable circular orbit at a given radius, derived by equating gravitational force to centripetal force. The WACE ATAR Unit 3 course requires Western Australian students to derive v = sqrt(GM/r) and apply it to satellite calculations.
- Kepler's Third Law
- The square of a planet's orbital period is proportional to the cube of its orbital semi-major axis (T squared proportional to r cubed). SCSA WACE exam questions require students to use this relationship to compare orbital parameters of planets and satellites.
- Gravitational Potential Energy
- The energy stored in a mass due to its position in a gravitational field, calculated as U = -GMm/r for point masses where the zero reference is at infinity. The SCSA WACE ATAR course requires students to explain the negative sign convention and calculate energy changes for orbital transfers.
- Escape Velocity
- The minimum speed required for an object to escape a gravitational field without further propulsion, calculated as v = sqrt(2GM/r). SCSA expects WACE ATAR students to derive this from energy conservation and apply it to different celestial bodies.
Sample Flashcards
Q1: State Newton's law of universal gravitation and define each variable.
F = GMm/r², where F is the gravitational force (N), G is the universal gravitational constant (6.674 × 10⁻¹¹ N m² kg⁻²), M and m are the masses (kg), and r is the centre-to-centre distance (m). The force is always attractive.
Q2: Define gravitational field strength and state its formula.
g = F/m = GM/r² (units: N kg⁻¹ or m s⁻²). At Earth's surface, g ≈ 9.80 N kg⁻¹. It is a vector directed toward the centre of the mass creating the field, independent of the test mass.
Q3: How does g vary with distance from a planet's centre?
g = GM/r² — an inverse-square law. Doubling r reduces g to one-quarter. At altitude h, r = R + h.
Q4: What are the two independent components of projectile motion?
Horizontal: constant velocity (aₓ = 0). Vertical: constant acceleration g = 9.80 m s⁻² downward. The two components are independent.
Q5: State the kinematic equations for projectile motion.
v = u + at, s = ut + ½at², v² = u² + 2as, s = ½(u + v)t. Apply separately to horizontal (a = 0) and vertical (a = ±g). Time links both components.
Q6: At what angle is range maximised on level ground?
45° gives maximum range (no air resistance). R = u² sin 2θ / g. Complementary angles (30° & 60°) give equal range.
Q7: Define uniform circular motion and the direction of centripetal acceleration.
Motion in a circle at constant speed. Velocity changes direction continuously, producing centripetal acceleration directed toward the centre, perpendicular to velocity.
Q8: State the formulas for centripetal acceleration and force.
a_c = v²/r = ω²r = 4π²r/T². F_c = mv²/r. Centripetal force is not a new force — it is the net inward force from real forces (gravity, tension, friction).
Sample Quiz Questions
Q1: Gravitational force is always attractive and acts along the line joining two masses.
Answer: TRUE
Every mass attracts every other mass. No gravitational repulsion exists.
Q2: Doubling the distance between two masses halves the gravitational force.
Answer: FALSE
F ∝ 1/r² — doubling r reduces F to one-quarter.
Q3: Gravitational field strength depends on the mass of the test object.
Answer: FALSE
g = GM/r² depends only on the source mass and distance.
Q4: N kg⁻¹ and m s⁻² are equivalent units for g.
Answer: TRUE
1 N = 1 kg m s⁻², so N kg⁻¹ = m s⁻².
Q5: In projectile motion without air resistance, horizontal velocity stays constant.
Answer: TRUE
No horizontal force means constant horizontal velocity.
Why It Matters
Gravitational fields and orbital mechanics represent some of the most elegant applications of physics, connecting Newton's universal gravitation to the motion of planets, satellites, and spacecraft. In the WACE Physics exam, gravity questions test your ability to apply mathematical relationships, interpret field diagrams, and solve multi-step orbital problems. Understanding gravitational field strength, potential energy, and Kepler's laws provides the conceptual framework for analysing everything from satellite orbits to tidal forces. This topic rewards students who develop strong problem-solving habits and can connect equations to physical meaning rather than substituting numbers mechanically. Gravity concepts connect to electromagnetism through the mathematical parallels between gravitational and electric field equations, reinforcing inverse-square law reasoning across both topics. Exam questions on gravity commonly require multi-step calculations combining Kepler's third law with gravitational field strength, so practise linking orbital period to radius and mass systematically.
Key Concepts
Newton's Law of Universal Gravitation
Every mass attracts every other mass with a force proportional to the product of their masses and inversely proportional to the square of their separation. Apply this law to calculate gravitational forces, understand the meaning of the gravitational constant G, and recognise that gravity is always attractive and acts at a distance.
Gravitational Field Strength and Potential Energy
Gravitational field strength (g) represents force per unit mass and decreases with distance from the source. Gravitational potential energy in a field is negative and approaches zero at infinity. Understand how to calculate work done moving masses in gravitational fields and relate field strength to the gradient of potential.
Orbital Motion and Satellite Physics
Satellites maintain orbit when gravitational force provides the centripetal acceleration needed for circular motion. Derive the relationship between orbital speed, radius, and period. Understand geostationary orbits, low Earth orbits, and the energy requirements for launching and transferring between orbits.
Kepler's Laws of Planetary Motion
Kepler's three laws describe planetary orbits: elliptical paths with the Sun at one focus, equal areas swept in equal times, and the period-radius relationship T squared proportional to r cubed. Apply these laws to solve problems involving planetary and satellite motion, and understand how Newton's gravitation explains Kepler's empirical laws.
Common Mistakes to Avoid
- Using the surface value of g (9.8 m/s squared) at orbital altitudes — the SCSA WACE ATAR course requires students to calculate gravitational field strength at the actual orbital radius using g = GM/r squared, which decreases with distance from the centre of mass.
- Forgetting that r in gravitational equations is measured from the centre of the mass, not from its surface — WACE examiners expect students to add the body's radius to the altitude above the surface when calculating orbital parameters.
- Confusing Kepler's third law constant between different central bodies — the proportionality constant T squared/r cubed depends on the central mass, so the value for Earth's satellites differs from that for the Sun's planets; SCSA WACE marking guides test this understanding.
- Omitting the negative sign in gravitational potential energy calculations — the SCSA WACE ATAR course requires students to understand that GPE is negative (bound system) and that energy must be added to move a mass further from the gravitational source.
Study Tips
- Derive the orbital speed and period equations from Newton's gravitation and circular motion — understanding the derivation helps you adapt to unfamiliar exam questions.
- Use flashcards with spaced repetition to memorise key gravitational equations and their conditions of use, including when each formula applies and its limitations.
- Practise sketching gravitational field lines and equipotential surfaces around single masses and binary systems to build physical intuition.
- Work through past WACE problems involving Kepler's third law, paying careful attention to unit conversions between AU and metres, years and seconds.
- When solving gravity problems, always start by drawing a diagram showing forces, distances, and the direction of gravitational field — this prevents sign and direction errors.
- Before your exam, work through the practice questions in this set at least twice using spaced repetition. Testing yourself repeatedly is the most effective revision strategy for long-term retention.
Related Topics
Frequently Asked Questions
What does WACE Physics Unit 3 Gravity & Motion cover?
This topic covers Newton's law of universal gravitation (F = GMm/r²), gravitational field strength, projectile motion, uniform circular motion, centripetal force, orbital mechanics, Kepler's three laws, satellite motion, gravitational potential energy and escape velocity.
How many flashcards are in this set?
This free set contains 20 flashcards and 20 true/false quiz questions covering all key gravity and motion concepts, aligned to the SCSA WACE Physics ATAR syllabus.
Are these flashcards aligned to the WACE ATAR syllabus?
Yes — every flashcard and quiz question is mapped to SCSA syllabus content for WACE Physics ATAR Unit 3.
Last updated: March 2026 · 20 flashcards · 20 quiz questions · Content aligned to the SCSA Curriculum