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Gravitation
Physics
Useful for: OSSC RHT — Regular High School Teacher, OSEPA Upper Primary — Science, OSSC Science, OSSSC Govt TGT Science — PCM, SSB Lecturer (Degree Colleges), SSB TGT PCM
Overview
Gravitation is the attractive force between all masses; it keeps planets in orbit and gives objects weight.
Key points
- Newton’s law of gravitation: F = G m₁ m₂ / r², with G ≈ 6.67 × 10⁻¹¹ N m² kg⁻².
- Acceleration due to gravity on Earth is about 9.8 m/s².
- Weight = mg; mass stays constant everywhere, weight changes.
- An object weighs about one-sixth as much on the Moon as on Earth.
- Kepler’s laws: elliptical orbits, equal areas in equal times, T² ∝ r³.
- Escape velocity from Earth is about 11.2 km/s.
Explanation
- g decreases with altitude and depth and is slightly higher at the poles.
- Geostationary satellites orbit at about 36,000 km with a 24-hour period.
- Free fall: all objects fall with the same acceleration without air resistance.
- Tides are caused mainly by the Moon’s gravity.
- Doubling distance reduces force to one-quarter.
Step by step
- Identify the concept, law or fact asked.
- Recall the definition, formula, example or year.
- Apply it and eliminate mismatched options.
Examples
- A 60 kg person weighs about 588 N on Earth (60 × 9.8).
- On the Moon the same person weighs about one-sixth, about 98 N.
- Doubling the distance between two masses → force becomes one-quarter.
Traps and advanced points
- Trap: Saying mass changes on the Moon.
- Trap: Heavier objects fall faster (in vacuum they don’t).
- Trap: Wrong escape velocity.
Quick revision
- F ∝ 1/r².
- g ≈ 9.8 m/s²; Moon weight 1/6.
- Escape velocity 11.2 km/s.
Practice MCQs
The force with which the Earth pulls objects towards it is —
- friction
- gravity
- magnetism
- tension
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Answer: (b) gravity. Gravity gives objects weight.
Acceleration due to gravity on Earth is about —
- 6.67 m/s²
- 9.8 m/s²
- 11.2 m/s²
- 1.6 m/s²
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Answer: (b) 9.8 m/s². g ≈ 9.8 m/s².
A 60 kg person weighs about how much on Earth?
- 600 kg
- 6 N
- 60 N
- 588 N
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Answer: (d) 588 N. 60 × 9.8 = 588 N.
On the Moon, an object weighs about —
- six times
- double
- the same
- one-sixth
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Answer: (d) one-sixth. Moon’s g is about 1/6.
Mass of an object on the Moon —
- stays the same
- doubles
- becomes zero
- becomes one-sixth
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Answer: (a) stays the same. Mass does not change.
If the distance between two masses doubles, the force becomes —
- half
- one-quarter
- double
- four times
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Answer: (b) one-quarter. Inverse square law.
Escape velocity from Earth is about —
- 7.9 km/s
- 11.2 km/s
- 3 km/s
- 30 km/s
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Answer: (b) 11.2 km/s. 11.2 km/s.
Kepler’s third law states that —
- T ∝ r
- F ∝ 1/r
- orbits are circles
- T² ∝ r³
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Answer: (d) T² ∝ r³. Harmonic law.
The value of G is about —
- 3 × 10⁸
- 1.6 × 10⁻¹⁹
- 9.8
- 6.67 × 10⁻¹¹ N m² kg⁻²
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Answer: (d) 6.67 × 10⁻¹¹ N m² kg⁻². Universal constant.
In a vacuum, a feather and a coin dropped together —
- do not fall
- the coin lands first
- land together
- the feather lands first
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Answer: (c) land together. Same acceleration.
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