Build a planet
"Build a planet" is a free, interactive Class 11 Physics simulation for NEET and JEE, built on the NCERT section "Acceleration due to gravity" in Gravitation. Change a planet's mass, radius or density; g = GM/R² and a person's weight update, separating changes at fixed mass from fixed density. NEET has asked about this idea 2 times in 10 years of papers. JEE Main asked 39 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
From the lesson Acceleration due to gravity in Gravitation. Change the values and watch what happens.
What it shows
NCERT §7.5
Change a planet's mass, radius or density; g = GM/R² and a person's weight update, separating changes at fixed mass from fixed density.
Controls
4 to try
- Sliderradius 0.25–4 R_E
- Slidermass 0.1–10 M_E
- Togglehold mass / hold density
- Sliderperson's mass 30–90 kg
Try this
Keep the earth's density and find the radius at which g is half its earth value (R_E/2).
Common mistake
Confusing G with g; assuming a bigger planet always has bigger g.
In NEET
10 years of papers
Asked 2 times in 10 years of NEET.
See the past questions: Gravitation, Gravitation (chapter)
In JEE Main
42 shifts analysed
39 questions from Gravitation across the 42 JEE Main shifts Lumi has analysed (2017–2026).
The idea behind it
NCERT §7.5
- Think of the earth as many concentric shells. A point outside is outside every shell, so the whole earth pulls as if its mass M_E were at the centre.
- On the surface the force on a mass m is F = G M_E m / R_E², and since F = mg, g = G M_E / R_E².
- g does not depend on the mass of the falling body; it depends only on the mass and radius of the earth.
- Knowing g, R_E and G gives the earth's mass: M_E = g R_E² / G. With g = 9.81 m/s² and R_E = 6.37 × 10⁶ m this is 5.97 × 10²⁴ kg, hence the phrase 'Cavendish weighed the earth'.
- The Moon's orbit gives the same mass another way, M_E = 4π²R³/(G T²) ≈ 6.02 × 10²⁴ kg, within 1% of the first value.
- Inside a uniform earth, at distance r from the centre, only the mass within radius r pulls, and the force is F = G M_E m r / R_E³, proportional to r.
Questions
5 answers
- What does the "Build a planet" simulation show?
- Change a planet's mass, radius or density; g = GM/R² and a person's weight update, separating changes at fixed mass from fixed density.
- What mistake does the "Build a planet" simulation fix?
- A common mistake it clears up: confusing G with g; assuming a bigger planet always has bigger g.
- Has NEET asked about this idea?
- Yes. NEET has asked about this idea 2 times in 10 years of papers.
- Does JEE Main ask questions from Gravitation?
- Yes. JEE Main asked 39 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
- Is this simulation free?
- Yes. It is free and runs in the browser on a phone or a laptop. It belongs to the lesson "Acceleration due to gravity" in Gravitation (Class 11 Physics).
Embed this simulation
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Teaching this idea? Put the simulation on your own page or LMS with this code.