Science lesson plan · 12th grade (Year 13 in England) · 60 minutes
Newton's law of gravitation and field strength
Forces and motion
CCSS NGSS.HS-PS2-4England: A level Physics: Gravitational fields (extension)
Objective and success criteria
Students will: Newton's law of gravitation and field strength.
- I can answer questions on today's skill on my own, without help.
- I can explain each step and say why it works.
- I can check my answer and spot a mistake.
Before and after
Needs first: Mass and weight (W = mg); Orbits and orbital speed (v = 2πr / T)
Leads to: Coulomb's law: force between charges
Materials: Force meters, Toy cars and ramps, Mini whiteboards, A diagram or short video clip of the idea, Printed student sheet (below).
Warm-up · 6 min
- Quick retrieval from earlier learning (Mass and weight (W = mg); Orbits and orbital speed (v = 2πr / T)). Students answer on mini whiteboards.
- Show answers together and note who needs support in the main task.
- What is the weight of a 60 kg object on Earth? Use g = 9.8 N/kg. → 588 NPrompt: weight = mass × gravitational field strength (W = m g)
- A geostationary satellite orbits at a radius of about 42160 km from Earth's centre, taking 1436 minutes per orbit. What is its orbital speed, in km/s? (v = 2πr ÷ T; give 1 decimal place) → 3.1 km/sPrompt: Convert the period to seconds.
- An astronaut's mass is 60 kg. Gravitational field strength on Mars is 3.7 N/kg. What would their weight be there, in N? → 222 NPrompt: Mass stays the same everywhere; weight changes.
Hook · 6 min
- Drop a sheet of paper and a scrunched-up sheet together and ask why one falls faster.
- Share the objective: Newton's law of gravitation and field strength. Read the success criteria together.
I do: model it · 12 min
- Work through the example out loud, saying why you do each step, not just what you do.
- Example: One of two masses is multiplied by 2 and the distance stays the same. By what factor does the gravitational force change? (F = G m₁ m₂ ÷ r²)
- Step 1: × 2
- Step 2: So the answer is: 2
- Answer: 2
- Pause after each step and ask a student to say what comes next.
We do: guided practice · 12 min
- Do these together. Students try each step on whiteboards before you reveal it.
- Use the hints as prompts rather than giving the answer.
- The distance between two masses is multiplied by 2. By what factor does the gravitational force between them change? (F = G m₁ m₂ ÷ r²; give 4 decimal places if needed) → 0.25Prompt: Force is proportional to each mass.
- Calculate the gravitational field strength at the surface of Earth, in N/kg. Mass = 5.97 × 10²⁴ kg, radius = 6.37 × 10⁶ m, G = 6.67 × 10⁻¹¹ N m²/kg². (g = G M ÷ r²; give 1 decimal place) → 9.8 N/kgPrompt: Square the radius first.
You do: independent practice · 17 min
- 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 8 questions are on the student sheet below, with answers in the key.
Exit ticket · 7 min
- Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
- Calculate the gravitational field strength at the surface of Earth, in N/kg. Mass = 5.97 × 10²⁴ kg, radius = 6.37 × 10⁶ m, G = 6.67 × 10⁻¹¹ N m²/kg². (g = G M ÷ r²; give 1 decimal place) → 9.8 N/kgPrompt: Square the radius first.
- Why do all objects near Earth's surface fall with the same acceleration (ignoring air)? (A larger mass feels a larger force, but also needs more force to accelerate, so a = g for all / Heavier objects feel no extra force / Gravity does not depend on mass / Light objects are pulled harder) → A larger mass feels a larger force, but also needs more force to accelerate, so a = g for allPrompt: W = m g and F = m a.
Watch out for
- Everyday meanings of words getting in the way of the science meaning: define key words explicitly.
- Remembering facts without the reason: ask "why?" and "what is the evidence?"
Support and stretch
Support
- Review first: Mass and weight (W = mg).
- Use concrete materials: force meters.
- Give the first step of each question, or a partly worked example to finish.
- Pair with a confident partner for the guided practice.
Stretch
- Move on to: Coulomb's law: force between charges.
- Explain a common mistake in writing and how to fix it.
- Write a word problem that uses today's skill, then solve a partner's.
Homework and more practice
Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-12.
Free lesson plan from eurekami.com/lesson-plan-builder