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7.0Gravitation

Overview: Gravitation: Planetary Motion and Using the Law of Gravitation — American System Physics Revision Notes & Topical Questions

5 min read 7 parts Lesson 2 of 5 Unit quiz

📚 American System Physics Hub / Topical Revision
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Lesson summary

State Kepler's laws.Planets move faster when closer to the Sun (second law).Astronauts in orbit are not beyond gravity; they are falling freely.Cavendish measured G with a torsion balance.Einstein described gravity as curvature of space-time.g decreases with altitude.State Kepler…

  • If the distance between two masses triples, what happens to the force?
  • Kepler's first law
  • Kepler's second law
  • Kepler's third law
  • Law of universal gravitation
  • Gravitational field

Section 1 of 7Overview: Gravitation: Planetary Motion and Using the Law of Gravitation

1
7.1 Gravitation: Planetary Motion and Usi… concept map: If the distance between two m, Kepler's first law, Kepler's second law, Kepler's third law, Law of universal gravitation
Visual guide to the key ideas explained in this lesson.
2
Tides result from differences in the Moon's gravity across Earth.
3
Geostationary satellites have a 24-hour period.
4
Escape velocity from Earth is about 11.2 km/s.
5
Gravity is always attractive.
6
r is measured from centres, not surfaces.

Section 2 of 7Process & sequence

1
Identify the two masses and the centre-to-centre distance.
2
Use F = Gm1m2/r².
3
For orbits, set gravity = centripetal force.
4
Solve for v or T: T = 2π√(r³/GM).
5
Check with Kepler's third law ratios.

Orbit conditionwrite these steps in this order; each step is a marking point.

  1. Gravity provides Fc
  2. GMm/r² = mv²/r
  3. v = √(GM/r)
  4. T = 2πr/v
  5. Apply Kepler III

Section 3 of 7Compare

Inverse-square law
DistanceForce
rF
2rF/4
3rF/9
r/24F
Orbit facts
QuantityDepends on
Orbital speedCentral mass and radius (not satellite mass)
PeriodRadius (T² ∝ r³)
Weightlessness in orbitContinuous free fall

Section 4 of 7Worked examples

  1. If the distance between two masses triples, what happens to the force?

Section 5 of 7Key terms

Section 6 of 7Command terms

Command terms
Identify / Describe (FRQ)
A short, specific statement; describing requires relevant characteristics, not a single word.
Explain / Justify (FRQ)
Give the cause-and-effect reasoning or evidence; a restated fact does not score.
Calculate
Show setup, substitution and the answer with units.
Predict
State the expected outcome; add a reason only if asked to justify.
MCQ technique
Read every option; eliminate distractors that are true statements but do not answer the question; watch for NOT/EXCEPT.

Section 7 of 7Exam technique

Keywords the mark scheme looks for in this lesson

Keywords the mark scheme looks for in this lesson
Kepler's first law · Kepler's second law · Kepler's third law · Law of universal gravitation · G · Gravitational field · Orbital speed · Inertial vs gravitational mass
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

1
Answer exactly what is asked; extra correct material does not earn credit, and contradictory statements can cancel credit (AP FRQ).
2
Use precise vocabulary (e.g. “active site”, “concentration gradient”, “allele”), never “it/stuff/things”.
3
Show units and working in calculations.
4
In MCQs, predict the answer before reading the options, then eliminate.

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