QCE Physics · Unit 3
QCE Physics Unit 3 Topic 1: Gravity & Motion — Flashcards & Quiz
QCE Physics Unit 3 Topic 1 explores how gravity governs motion from everyday projectiles to orbiting satellites. These free flashcards and true/false questions cover Newton's law of universal gravitation (F = GMm/r²), gravitational field strength, projectile motion analysis, uniform circular motion and centripetal acceleration, Kepler's three laws of planetary motion, satellite orbits, and gravitational potential energy. Every card is aligned to the QCAA Senior Physics syllabus so you revise exactly what examiners assess. Build long-term recall of inverse-square relationships, orbital mechanics and energy transformations through spaced repetition.
Key Terms
- Gravitational field strength (g)
- The force per unit mass experienced by a test mass at a point in a gravitational field, calculated as g = GM/r squared. QCAA Physics Unit 3 Topic 1 EA questions require students to calculate g at different altitudes and explain why it decreases with increasing distance from the centre of the mass.
- Coulomb's law
- The equation F = kq1q2/r squared describing the electrostatic force between two point charges, where k is the Coulomb constant. QCAA external assessments test whether students correctly apply the sign convention for attractive (opposite charges) and repulsive (like charges) forces.
- Inverse square law
- The mathematical relationship where field strength or force is inversely proportional to the square of the distance from the source. QCAA Physics Unit 3 Topic 1 assessments apply this principle to both gravitational and electrostatic fields and may ask students to compare the two field types.
- Gravitational potential energy
- The energy stored in a mass due to its position in a gravitational field, calculated as Ep = negative GMm/r for point masses. QCAA EA questions require students to explain why this value is always negative and apply energy conservation to satellite orbit and escape velocity problems.
- Geostationary orbit
- A circular orbit directly above the equator with a period of exactly 24 hours, so the satellite appears stationary relative to Earth's surface. QCAA Physics Unit 3 Topic 1 assessments may ask students to calculate the orbital radius by equating gravitational force with centripetal force and setting the period to 24 hours.
- Electric field strength (E)
- The force per unit positive test charge at a point in an electric field, calculated as E = kQ/r squared for a point charge or E = V/d for uniform parallel plates. QCAA EA questions test both formulas and require students to draw field line diagrams with correct line direction and spacing.
Sample Flashcards
Q1: State Newton's law of universal gravitation.
F = GMm/r², where F is gravitational force (N), G = 6.67 × 10⁻¹¹ N m² kg⁻², M and m are masses (kg), r is centre-to-centre distance (m). Always attractive.
Q2: Define gravitational field strength.
g = F/m = GM/r² (N kg⁻¹ or m s⁻²). Force per unit mass at a point in a gravitational field. At Earth's surface g ≈ 9.8 N kg⁻¹. Vector directed toward the source mass.
Q3: How does g vary with distance from a planet?
g = GM/r² — inverse-square law. Doubling r quarters g. At altitude h, use r = R + h. Field strength decreases with altitude but never reaches zero.
Q4: Describe the two independent components of projectile motion.
Horizontal: constant velocity (a = 0), x = v_x t. Vertical: constant acceleration g = 9.8 m s⁻² downward. Components are independent — horizontal does not affect vertical.
Q5: Find range and max height for a projectile at angle θ.
v_x = v cos θ, v_y = v sin θ. H = v_y²/(2g). T = 2v_y/g. R = v² sin 2θ / g. Max range at 45°.
Q6: How do you find velocity at any point during flight?
v = √(v_x² + v_y²), v_y = v_y₀ − gt. Direction: tan θ = |v_y|/v_x below horizontal.
Q7: Define centripetal acceleration.
In uniform circular motion, velocity changes direction requiring a_c = v²/r = ω²r toward the centre. Speed is constant but velocity is not.
Q8: What provides centripetal force?
F_c = mv²/r is not a new force — it is the real force acting inward: tension, friction, gravity or normal force.
Sample Quiz Questions
Q1: Gravitational force is proportional to the product of the two masses.
Answer: TRUE
F = GMm/r² — F ∝ Mm.
Q2: Tripling the distance reduces gravitational force to one-third.
Answer: FALSE
Inverse-square: F reduces by 3² = 9.
Q3: g depends on the test mass placed in the field.
Answer: FALSE
g = GM/r² — independent of test mass.
Q4: N kg⁻¹ is equivalent to m s⁻² for g.
Answer: TRUE
1 N = 1 kg m s⁻², so N/kg = m s⁻².
Q5: Horizontal velocity stays constant without air resistance.
Answer: TRUE
No horizontal force → no horizontal acceleration.
Why It Matters
Gravity and electrostatics is the opening topic of QCE Physics Unit 3, establishing the field model framework that extends through electromagnetism and into Unit 4. The external exam places significant emphasis on quantitative problem-solving — you must be confident applying Newton's law of universal gravitation and Coulomb's law, calculating field strengths and potentials, and interpreting field diagrams. These concepts also underpin satellite motion and orbital mechanics, which are frequently tested through data-analysis questions involving real satellite data. The field model framework introduced here carries directly into Topic 2's electromagnetism content, where electric and magnetic fields combine to explain electromagnetic induction. QCAA exam questions commonly require you to calculate gravitational or electric field strength at a specific point, then determine the force on a test mass or charge placed there, so practise chaining these calculations fluently.
Key Concepts
Gravitational Fields and Newton's Law
Apply F = GMm/r^2 to calculate gravitational force between masses. Understand gravitational field strength (g = GM/r^2) as force per unit mass and how it varies with altitude. Draw gravitational field lines for point masses and uniform fields near Earth's surface, noting that field lines always point toward the mass.
Electrostatic Fields and Coulomb's Law
Apply F = kq1q2/r^2 for electrostatic force calculations, noting the sign convention for attractive and repulsive forces. Calculate electric field strength (E = F/q = kQ/r^2) and draw field line diagrams for point charges and parallel plates. Understand the uniform field between parallel plates (E = V/d).
Gravitational and Electric Potential Energy
Calculate gravitational potential energy (Ep = -GMm/r) and electric potential energy (Ep = kq1q2/r). Understand why gravitational PE is always negative and explain the significance of escape velocity. Practise energy conservation problems involving satellites and charged particles.
Satellite Motion and Orbits
Derive orbital velocity and period for circular orbits by equating gravitational force with centripetal force. Understand the difference between low Earth orbits and geostationary orbits. Be able to calculate orbital parameters from given data and explain why astronauts experience apparent weightlessness in orbit.
Common Mistakes to Avoid
- Using the wrong distance in inverse square law calculations — r is measured from the CENTRE of the mass or charge, not from the surface. QCAA Physics Unit 3 Topic 1 EA questions may provide the radius of Earth and the altitude above the surface, requiring students to add both to find r.
- Forgetting the negative sign in gravitational potential energy (Ep = negative GMm/r) — QCAA marking rubrics deduct marks if students present gravitational PE as positive, since the negative sign indicates a bound system where work must be done to separate the masses.
- Confusing gravitational field (always attractive) with electric field (attractive or repulsive) — QCAA assessments test this distinction directly, and students must note that gravitational field lines always point toward the mass, while electric field lines point away from positive charges and toward negative charges.
- Substituting values without converting units to SI (metres, kilograms, seconds, coulombs) — QCAA Physics EA calculation questions present data in mixed units (km, grams) to test whether students convert before substituting into equations.
Study Tips
- Practise substituting values into inverse square law equations (gravity and Coulomb's) until unit conversion and significant figures become automatic.
- Draw field line diagrams for single charges, dipoles, parallel plates and point masses from memory — examiners award marks for correct line direction, spacing and density.
- Solve satellite orbit problems by first writing the force balance equation (gravitational = centripetal), then algebraically isolating the unknown before substituting numbers.
- Compare gravitational and electrostatic fields in a side-by-side table noting similarities (inverse square law) and differences (attractive only vs both).
- Use flashcards with spaced repetition to drill equations, units and field diagram conventions — rapid recall of formulas like g = GM/r^2 and E = kQ/r^2 prevents costly errors under exam time pressure.
- 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 QCE Physics Unit 3 Topic 1 cover?
Unit 3 Topic 1 covers Newton's law of universal gravitation, gravitational field strength, projectile motion, uniform circular motion, centripetal acceleration, Kepler's laws, satellite orbits, and gravitational potential energy.
How many flashcards are in this set?
This free set contains 20 flashcards and 20 true/false quiz questions covering all key concepts in Unit 3 Topic 1, aligned to the QCAA Senior Physics syllabus.
Are these flashcards aligned to the Queensland QCE syllabus?
Yes — every flashcard and quiz question is mapped to QCAA syllabus objectives for QCE Physics Unit 3 Topic 1: Gravity and Motion.
Last updated: March 2026 · 20 flashcards · 20 quiz questions · Content aligned to the QCAA Syllabus