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6.1.7Space Physics

Elliptical Orbits — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 9 parts Lesson 12 of 24 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

6.1.7 Elliptical Orbits — IGCSE Physics Notes — IGCSE PHYSICS notes: Describe elliptical orbits and explain how the speed, radius and energy of an orbiting body change round them.Ellipse — a squashed circle that has two foci.Planets and minor planets — their orbits are only sligh…

  • Circular case
  • Calculations
  • Stable orbit
  • Closer to the Sun
  • Further from the Sun
  • Fastest point

Part 1 of 7Shape of orbits

Shape of orbits
Ellipse
  • Ellipse a squashed circle that has two foci.
Planets and minor planets
  • Planets and minor planets their orbits are only slightly elliptical and approximately circular.
  • The Sun is generally taken to be at the centre.
Comets
  • Comets they have highly elliptical orbits.
  • The Sun is not at the centre but at one of the two foci.
Circular case
  • Circular case a circle is an ellipse in which the two foci are at the same place.
Calculations
  • Calculations questions about period, speed or radius assume the orbit is circular.

Part 2 of 7Changing speed

Stable orbit
  • Stable orbit an elliptical orbit is stable even though the speed changes.
Closer to the Sun
  • Closer to the Sun the orbital radius decreases and the speed increases because of the stronger pull.
Further from the Sun
  • Further from the Sun the orbital radius increases and the speed decreases because of the weaker pull.
Fastest point
  • Fastest point the closest approach to the Sun (perihelion).
Slowest point
  • Slowest point the furthest point from the Sun (aphelion).

Part 3 of 7Why speed changes

Why speed changes
Gravity pull
  • Gravity pull gravity weakens with distance, so the force is greatest at the closest point.
Component of force
  • Component of force along an ellipse gravity has a component along the direction of motion.
  • Falling towards the Sun it speeds the body up.
  • Moving away it pulls backwards and slows the body down.

Part 4 of 7Energy in an elliptical orbit

Conservation
  • Conservation total energy is conserved, with gravitational potential energy and kinetic energy exchanged.
Approaching the Sun
  • Approaching the Sun the body loses gravitational potential energy and gains kinetic energy, so it speeds up.
Moving away
  • Moving away the body gains gravitational potential energy and loses kinetic energy, so it slows down.
Kinetic energy
  • ½mv², so a larger speed means a larger kinetic energy.
Circular orbit
  • Circular orbit the distance is constant, so neither form changes.

Part 5 of 7Comets

Composition
  • Composition a frozen lump of ice and dust.
Time spent
  • Time spent most of the time is spent far from the Sun, moving slowly.
Near the Sun
  • Near the Sun the ice evaporates and a tail forms that points away from the Sun.
Example
  • Halley's comet takes about 76 years and travels out beyond the orbit of Neptune.

Part 6 of 7Planets compared with comets

FeaturePlanetComet
Orbit shapeNearly circularLong, thin ellipse
Sun's positionNear the centreAt one focus
SpeedAlmost constantLarge changes

Part 7 of 7More from the specification

More from the specification
Earth's orbit
  • Earth's orbit it is slightly elliptical, but this does not cause the seasons or large temperature changes;
    • the tilt of the axis does.
Earth's orbit
  • Earth's orbit it is slightly elliptical, but this does not cause the seasons or large temperature changes;
    • the tilt of the axis does.
Hemisphere
  • Half of the Earth, northern or southern.
Nebula
  • Cloud of gas and dust in which stars form.
Solar system
  • A star with the objects that orbit it.
Gravitational field
  • Region where mass experiences gravitational force.
Field strength
  • G = Fg/m.
Artificial satellite
  • A man-made object orbiting a body in space.
Earth's orbit
  • Earth's orbit it is slightly elliptical, but this does not cause the seasons or large temperature changes;
    • the tilt of the axis does.

Key terms

TermMeaning
EllipseA closed oval curve with two foci.
FocusOne of the two special points of an ellipse; the Sun is at one.
PerihelionThe point of an orbit closest to the Sun.
AphelionThe point of an orbit furthest from the Sun.
Gravitational potential energyEnergy stored in a gravitational field, which increases as a body moves away from the Sun.

Avoid these mistakes

1
Putting the Sun at the centre of an elliptical orbit — it is at one focus.
2
Saying a comet is fastest when far from the Sun — it is fastest when closest.
3
Describing planetary orbits as very elongated — they are nearly circular.
4
Saying the comet's tail trails behind it — the tail points away from the Sun.

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