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CBSE Class 11 · Physics

Gravitation

Official NCERT chapter from Physics Part I–II (book code keph1). ExamMaster notes are original teaching at CBSE Class 11 depth.

This lesson follows the official chapter “Gravitation” in Physics Part I–II. The words below are ExamMaster’s teaching, not a paste from the book. Use the NCERT chapter for the classroom sequence; use these notes to hold the idea without copying exercises or figures.

  • CBSE Class 11
  • Medium level
  • 8 concepts

1Kepler’s laws

Kepler’s laws (school form): orbits are ellipses with the Sun at a focus; equal areas in equal times; T² ∝ a³ for the taught solar family. A circle is a special ellipse. The laws are look-summaries that Newton’s inverse-square later explains — do not invent extra planets.

A perfect everyday circle as “the only Kepler orbit” skips the ellipse.

Figure. T squared tracks a cubed for the same constant. The first-law ellipse is not drawn.

How it works

  1. Name ellipse, equal-area, T²∝a³ as taughtThe three.
  2. Keep the Sun at a focus, not always the exact centre unless circularThe first.
  3. One planet-pair is enough for T²/a³This class.

2Universal law of gravitation

Universal gravitation: F = G m1 m2 / r² along the join, attractive. Two 10 kg a metre apart give a tiny F — G is small. Inverse-square means double r, quarter F. A “gravity only on Earth” story fails the universal word.

Using r as the radius of one ball when the lesson used centre-to-centre is a miss if that was the write.

Figure. Each mass pulls the other with the same F. F grows with both masses and falls as one over r squared.

How it works

  1. Name m1, m2, rThe given.
  2. Write G m1 m2 / r²The law.
  3. Keep it along the join, attractiveThe direction.

Double the separation

If F is 12 nN at r, what is F at 2r (same masses)?

  • Inverse square×1/4
  • F3 nN
  • ReadUniversal; G stayed

Pro tip. Double r, quarter F. Original 12 nN is a school size.

3The gravitational constant

G is the same small constant in the law — measured, not 10 m/s². g is a local field; G is the universal factor. Mixing G and g is the heading-steal. A Cavendish-story if taught is “how we learned G”, not a new law.

Writing F = g m1 m2 / r² is a letter-swap.

Figure. Double the separation and F falls to one quarter. G is the constant that does not change.

How it works

  1. Keep G in the 1/r² lawThe constant.
  2. Keep g as the local N/kg or m/s²The field.
  3. Refuse a swap of the lettersTwo meanings.

4Acceleration due to gravity of the earth

g on Earth’s surface is GM/R² as taught (or the school 9.8 or 10). A 2 kg weight is 20 N if g=10. g is not G. A different planet has a different g from its M and R.

Using 10 as G in the universal law is the swap again.

Figure. At the surface, g is GM over R squared. Earth is a block here; R is centre to rim, not a drawn sphere.

How it works

  1. Write g = GM/R² if asked, or the school gThe local.
  2. Then W = mgThe weight.
  3. Name Earth or the given planetWhose g.

5Acceleration due to gravity below and above the surface of earth

Above the surface, g(h) ≈ GM/(R+h)²; below, a taught linear drop with depth if the lesson used uniform-sphere. Higher is weaker; the centre-story is not “g=0 everywhere inside” unless taught that way. One height-case is enough.

Using the surface g at a satellite height without the (R+h) is a miss.

Figure. g rises linearly from the centre to the surface, then falls as one over r squared. Peak is at r = R.

How it works

  1. Name h or the depthThe given.
  2. Write the taught above/below formThe change.
  3. Keep surface g as the h=0 valueA compare.

6A definition is a test you can run

g / G / Kepler is a test: a named 1/r², a local g, or T²∝a³. If you only say “gravity”, you have a heading.

An apple-logo is not the test.

Figure. g is a test you can run: name the acceleration, write its SI unit, then write GM over R squared. A slogan is not the test.

How it works

  1. Name the two masses and r, or M,R, or T,aThe object.
  2. Give the writeThe test.
  3. Then the word has contentThe definition ran.

7Name the given before the unknown

The given are m1, m2, r (or M, R, h). The unknown is F or g. Copy metres before you use kilometres in r².

Using r=6400 because R is 6400 km, then squaring as if it were metres, is a silent unit.

Figure. Name Earth mass and radius before asking for g. The unknown sits after the named givens.

How it works

  1. Copy masses and r in SIThe given.
  2. Name F or gThe unknown.
  3. Then 1/r²Given first.

8One worked case is enough at this class

One inverse-square scale or one g=GM/R² is enough. A page of the same quarter-F clone does not add a new Kepler T².

Ten twins of 3 nN are still one idea.

Figure. One clean surface case is enough at this class: name M and R, write g = GM over R squared, keep SI units.

How it works

  1. Run one F-scale or one gThe case.
  2. A second only as a checkOptional.
  3. StopThis class.
Double r (same masses) makes F
  1. A quarter
  2. Double
  3. The same

Inverse square.

Notes

  • Mapped to the official NCERT chapter “Gravitation”. Original teaching only — no textbook sentences.
  • Science here is Physics, Chemistry and Biology ideas at this class, never a language or social-science chapter.

Formulas

  • F=G m1 m2 / r²
  • g=GM/R² (surface)
  • T² ∝ a³

Recap

Hold these pegs from the official chapter “Gravitation”. The wording is ExamMaster’s teaching, not a textbook recap.

Kepler’s laws
Kepler’s laws (school form): orbits are ellipses with the Sun at a focus; equal areas in equal times; T² ∝ a³ for the taught solar family.
Universal law of gravitation
Universal gravitation: F = G m1 m2 / r² along the join, attractive.
The gravitational constant
G is the same small constant in the law — measured, not 10 m/s².
Acceleration due to gravity of the earth
g on Earth’s surface is GM/R² as taught (or the school 9.8 or 10).
Acceleration due to gravity below and above the surface of earth
Above the surface, g(h) ≈ GM/(R+h)²; below, a taught linear drop with depth if the lesson used uniform-sphere.
A definition is a test you can run
g / G / Kepler is a test: a named 1/r², a local g, or T²∝a³.

Practise Gravitation

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