The shape of the lesson
By the end of the lesson, learners can
- Ride the ellipse: Follow a comet round its elliptical orbit and watch its speed change with distance.
- Energy traded round the orbit: Kinetic and gravitational potential energy convert into each other as the distance changes.
- Closest: fastest, most kinetic energy.
- Furthest: slowest, most gravitational energy.
- Total: KE + GPE is constant.
One picture every idea returns to
Orbits are ellipses. As an orbiting body moves closer to the Sun it speeds up, and as it moves further away it slows down, because gravitational potential energy and kinetic energy convert into each other while the total energy stays the same.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Faster when close.. Orbits are not tidy circles but stretched ellipses, and a comet feels it. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Ride the ellipse | Follow a comet round its elliptical orbit and watch its speed change with distance. Project tab 4 of the lab (Elliptical orbits) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Energy traded round the orbit | Kinetic and gravitational potential energy convert into each other as the distance changes. | Think-Pair-Share |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 13 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Energy traded round the orbit, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Think-Pair-Share
Pose the question, give thirty seconds of silent thinking, then a minute to compare with a partner before a few pairs share with the class. The silent minute is the part that cannot be cut.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| An orbiting body moves fastest when it is closest to the Sun and slowest when furthest away. | As it falls inward, gravitational potential energy becomes kinetic energy, and the reverse happens as it climbs back out. |
Elliptical orbits and energy: worksheet
6 questions, 13 marks. Written in the style of the papers; no past paper question is reproduced.
Describe the shape of a comet's orbit and state where the Sun is in it.
State where in its orbit a comet moves fastest, and where it moves slowest.
As a comet moves away from the Sun, describe what happens to its kinetic energy, its gravitational potential energy, and its total energy.
Explain, using energy, why a comet speeds up as it approaches the Sun.
At which point in its orbit does a comet have the greatest kinetic energy, and at which point the greatest gravitational potential energy?
A student says the comet must be gaining energy from somewhere to speed up near the Sun. Explain why this is not correct.
Answer key and marking notes
- A very stretched ellipse. ✓
- The Sun is at one focus of the ellipse. ✓
- Fastest when closest to the Sun. ✓
- Slowest when furthest from the Sun. ✓
- Kinetic energy decreases (it slows down). ✓
- Gravitational potential energy increases. ✓
- Total energy stays the same. ✓
- Its gravitational potential energy decreases as it gets closer. ✓
- That energy becomes kinetic energy, so its speed increases (total energy is conserved). ✓
- Greatest kinetic energy at the closest point to the Sun. ✓
- Greatest gravitational potential energy at the furthest point. ✓
- The total energy does not change. ✓
- The comet simply converts gravitational potential energy into kinetic energy. ✓
3 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
A comet moves in a very elliptical orbit around the Sun. At which point does it move fastest?
- Awhen it is closest to the Sun
- Bwhen it is farthest from the Sun
- Cits speed is the same all the way around
- Dexactly halfway between closest and farthest
As a comet moves away from the Sun, it slows down. In terms of energy, what is happening?
- Akinetic energy is transferred to gravitational potential energy
- Bgravitational potential energy is transferred to kinetic energy
- Cenergy is destroyed as it slows
- Dthe total energy increases as it moves away
How do the orbits of the planets compare with the orbit of a typical comet?
- AThe planets have nearly circular orbits, while a comet has a very elongated (elliptical) orbit.
- BBoth have equally circular orbits.
- CThe planets have more elongated orbits than comets.
- DComets do not orbit the Sun at all.
Quiz key
A comet moves fastest when closest to the Sun, where the gravitational pull is strongest, and slowest when farthest away. Its speed changes continuously around the elliptical orbit.
As the comet moves away it gains gravitational potential energy and loses kinetic energy, so it slows down; the reverse happens as it falls back toward the Sun. The total energy stays constant rather than being created or destroyed.
Planets follow nearly circular orbits, staying at roughly the same distance from the Sun, whereas comets follow very elongated elliptical orbits that bring them close to the Sun and then far out into the Solar System.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.