What you’ll learn15 learning objectivesChoose one objective for a focused lesson, or study the complete topic.—D.1.1—Kepler’s three laws of orbital motion• Kepler’s three laws of orbital motion.Syllabus objective—D.1.2—Universal gravitation• Universal gravitation: F=Gm1m2/r^2 for point masses.Syllabus objective—D.1.3—Point-mass approximation• Extended bodies can be treated as point masses only under suitable conditions.Syllabus objective—D.1.4—Gravitational field strength• Gravitational field strength: g=F/m=GM/r^2.Syllabus objective—D.1.5—Gravitational field lines• Gravitational field lines.Syllabus objective—D.1.6 (HL)—Gravitational potential energy• Gravitational potential energy is work to assemble masses from infinite separation.Syllabus objective—D.1.7 (HL)—Two-body potential energy• Two-body gravitational potential energy: Ep=-Gm1m2/r.Syllabus objective—D.1.8 (HL)—Gravitational potential• Gravitational potential: Vg=-GM/r, zero at infinity.Syllabus objective—D.1.9 (HL)—Potential gradient• Field strength is potential gradient: g=-ΔVg/Δr.Syllabus objective—D.1.10 (HL)—Work in gravitational fields• Work in gravitational field: W=mΔVg.Syllabus objective—D.1.11 (HL)—Gravitational equipotentials• Equipotential surfaces for gravitational fields.Syllabus objective—D.1.12 (HL)—Equipotentials and field lines• Relationship between equipotential surfaces and gravitational field lines.• Equipotential surfaces are perpendicular to field lines.Syllabus objective—D.1.13 (HL)—Escape speed• Escape speed: vesc=sqrt(2GM/r).Syllabus objective—D.1.14 (HL)—Orbital speed• Circular orbital speed: vorbital=sqrt(GM/r).Syllabus objective—D.1.15 (HL)—Atmospheric drag on orbits• Small atmospheric drag changes both height and speed of an orbiting body.Syllabus objective