UPSC Prelims 2011 Q81: Science & Technology – Space Technology & Astronomy
An artificial satellite orbiting around the Earth does not fall down. This is so because the
attraction of Earth
- (a) does not exist at such distance
- (b) is neutralized by the attraction of the moon
- (c) provides the necessary speed for its steady motion
- (d) provides the necessary acceleration for its motion
Answer: (d) provides the necessary acceleration for its motion
Explanation
Why an orbiting satellite does not fall to Earth
This is a classic application of circular motion and gravity.
| Option | Verdict | Reason |
|---|---|---|
| (a) Earth's attraction does not exist at such distance | Incorrect | Gravity never becomes zero; it only weakens with distance, it never vanishes |
| (b) Neutralized by the moon's attraction | Incorrect | The Moon's gravitational pull is far too weak at a satellite's orbital distance to neutralize Earth's pull |
| (c) Provides the necessary speed for steady motion | Incorrect | Gravity provides acceleration (force), not directly 'speed'; this phrasing misdescribes the physics |
| (d) Provides the necessary acceleration for its motion | Correct | Earth's gravity supplies the centripetal acceleration that continuously curves the satellite's path into a stable orbit, instead of letting it fly off in a straight line or fall straight down |
Why satellites stay in orbit:
| Point | Detail |
|---|---|
| Force involved | Gravity, acting as centripetal force |
| Effect | Provides continuous inward acceleration |
| Result | Satellite follows a curved, stable orbital path |
Answer: (d) provides the necessary acceleration for its motion.
Why each option is right or wrong
- (a) Incorrect. Gravity never disappears with distance; it only weakens, following the inverse-square law.
- (b) Incorrect. The Moon's gravity is far too weak at orbital altitude to balance out Earth's pull.
- (c) Incorrect. Gravity's role is to provide acceleration/force, not 'speed' directly; this phrasing is imprecise physics.
- (d) CORRECT. Gravity supplies the centripetal acceleration that keeps the satellite curving around Earth in a stable orbit.
Key point
A satellite stays in orbit because Earth's gravity supplies the CENTRIPETAL ACCELERATION needed to continuously curve its path — gravity never vanishes, and it isn't 'neutralized' by the Moon.
Remember
Satellites orbit because Earth's gravity provides the centripetal acceleration for their curved motion — it never vanishes or gets neutralized.
Common mistakes
- Thinking gravity disappears entirely at orbital distances instead of just weakening.
- Confusing the role of gravity as acceleration/force with directly providing 'speed'.
- Imagining the Moon's pull is strong enough to cancel Earth's gravity on a satellite.
Exam tip
Orbiting satellites: gravity = centripetal acceleration, curving the path, never zero, never cancelled by the Moon.
Sources
- NCERT Class XI Physics — Gravitation (orbital motion of satellites)
- H. C. Verma — Concepts of Physics, Volume 1 (Gravitation)