Gravitation – Complete Intermediate Notes

Stage 1 • Page 2 • Gravitational Field & Acceleration due to Gravity


1. Gravitational Field

A gravitational field is the region around a mass in which another mass experiences a gravitational force.

Definition:
Gravitational field at a point is the gravitational force experienced by a unit mass placed at that point.

g = F / m

  • It is a vector quantity
  • Direction is towards the attracting mass
  • SI unit: N kg⁻¹

IIT Insight:
Gravitational field concept is used in advanced problems involving multiple masses.


2. Gravitational Field Due to a Point Mass

For a point mass M, gravitational field at distance r is:

g = GM / r²

  • Field decreases with square of distance
  • Same expression as acceleration due to gravity
  • Independent of test mass

Key Idea:
Gravitational field is a property of the source mass, not the test mass.


3. Acceleration Due to Gravity (g)

When a body falls freely under the influence of Earth’s gravitational force, it experiences an acceleration called acceleration due to gravity.

g = GM / R²

  • M → Mass of Earth
  • R → Radius of Earth
  • SI unit: m s⁻²

Average value of g on Earth’s surface:

g ≈ 9.8 m s⁻²


4. Difference Between g and G

G g
Universal constant Acceleration
Same everywhere Varies with place
Very small Relatively large

5. Important Properties of g

  • Independent of mass of falling body
  • Acts vertically downward
  • Varies with altitude, depth, latitude
  • Maximum at poles, minimum at equator

Board Exam Tip:
Questions often ask conceptual reasons for variation of g.


6. Common Student Mistakes

  • Confusing g with G
  • Using surface formula inside Earth
  • Wrong units for g
  • Assuming g constant everywhere

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