• Quick retrieval from earlier learning (Earth's orbit, years and seasons; Scalars and vectors: distance vs displacement, speed vs velocity). Students answer on mini whiteboards.
• Show answers together and note who needs support in the main task.

1. What causes the seasons? (the Moon blocking sunlight / the Sun getting hotter and colder / the tilt of Earth's axis as it orbits the Sun / Earth getting closer to and further from the Sun) → the tilt of Earth's axis as it orbits the SunPrompt: Earth is actually closest to the Sun in early January.
2. Is speed a scalar or a vector quantity? (scalar (size only) / vector (size and direction)) → scalar (size only)Prompt: Does it make sense to give it a direction?
3. A year is 365 days, plus one extra day every 4 years (a leap year). How many days are there in 20 years in a row? → 7305Prompt: How many leap days are there in 20 years?

• Ask why we see the Moon change shape over a month.
• Share the objective: Orbits and orbital speed (v = 2πr / T). Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: A polar weather satellite orbits at a radius of about 7200 km from Earth's centre, taking 101 minutes per orbit. What is its orbital speed, in km/s? (v = 2πr ÷ T; give 1 decimal place)
• Step 1: T = 101 × 60 = 6060 s
• Step 2: 2π × 7200 = 45239 km
• Step 3: 45239 ÷ 6060 = 7.47
• Step 4: So the answer is: 7.5 km/s
• Answer: 7.5 km/s
• Pause after each step and ask a student to say what comes next.

• Do these together. Students try each step on whiteboards before you reveal it.
• Use the hints as prompts rather than giving the answer.

1. A polar weather satellite orbits at a radius of about 7200 km from Earth's centre, taking 101 minutes per orbit. What is its orbital speed, in km/s? (v = 2πr ÷ T; give 1 decimal place) → 7.5 km/sPrompt: Convert the period to seconds.
2. How does orbital speed change for a smaller (lower) stable orbit? (It stops / It must move slower / It must move faster / Speed does not depend on radius) → It must move fasterPrompt: Gravity is stronger closer to Earth.

• Students work on the student sheet. Circulate and check the first two answers of every student early.
• Students who finish: write a question of their own for a partner, with the answer.

The 6 questions are on the student sheet below, with answers in the key.

• Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.

1. Orbits: which statement is NOT true? (Gravity provides the force that keeps planets and satellites in orbit. / Astronauts on the ISS feel weightless because they are in free fall around Earth. / The force on an orbiting object points towards the centre of the orbit. / There is no gravity at the height of the ISS.) → There is no gravity at the height of the ISS.Prompt: Is velocity the same as speed?
2. A satellite moves at a steady speed in a circular orbit. Why is it accelerating? (Its engines are always on / It is not accelerating / Its speed increases every orbit / Its direction is always changing, so its velocity changes) → Its direction is always changing, so its velocity changesPrompt: Velocity is a vector.

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-11.