Physics Gravity and Gravitation 2 — Questions and Answers
Question 1: Kepler's Second Law (Law of Equal Areas) states that a planet:
- Moves faster when farther from the Sun
- Sweeps out equal areas in equal time intervals as it orbits (Correct answer)
- Has an orbital period proportional to its distance from the Sun
- Orbits the Sun in a perfect circle
Correct answer: Sweeps out equal areas in equal time intervals as it orbits
Kepler's Second Law means a planet moves faster when near the Sun and slower when far away, always sweeping equal areas in equal times.
Question 2: Kepler's Third Law states that the square of a planet's orbital period is proportional to:
- Its orbital radius
- The inverse square of its orbital radius
- The cube of its semi-major axis (Correct answer)
- The square of its orbital eccentricity
Correct answer: The cube of its semi-major axis
Kepler's Third Law: T² ∝ a³, linking the orbital period T to the semi-major axis a of the orbit.
Question 3: Escape velocity is defined as the minimum speed needed to:
- Achieve a stable circular orbit around a planet
- Break free from a planet's gravitational field without further propulsion (Correct answer)
- Travel from one planet to another in the solar system
- Maintain a stable elliptical orbit
Correct answer: Break free from a planet's gravitational field without further propulsion
Escape velocity is the minimum launch speed required to escape a gravitational field entirely, given by v = √(2GM/r).
Question 4: Earth's escape velocity is approximately:
- 7.9 km/s
- 11.2 km/s (Correct answer)
- 9.8 km/s
- 3.0 km/s
Correct answer: 11.2 km/s
Earth's escape velocity is approximately 11.2 km/s, calculated using v = √(2GM/R) with Earth's mass and radius.
Question 5: In free fall, objects of different masses (ignoring air resistance):
- Fall at rates proportional to their mass
- All accelerate at the same rate g (Correct answer)
- Lighter objects fall faster
- Fall at rates proportional to their weight
Correct answer: All accelerate at the same rate g
In free fall, all objects accelerate at g ≈ 9.8 m/s² regardless of mass, as famously demonstrated by Galileo.
Question 6: A satellite in low Earth orbit completes one orbit in approximately:
- 24 hours
- 12 hours
- 90 minutes (Correct answer)
- 6 hours
Correct answer: 90 minutes
Low Earth orbit satellites at altitudes of 300–400 km complete one full orbit in roughly 90 minutes.
Question 7: The gravitational field strength g at a height h above Earth's surface is:
- Greater than at the surface
- Equal to the surface value regardless of height
- Less than at the surface (Correct answer)
- Zero everywhere above the atmosphere
Correct answer: Less than at the surface
Since g = GM/r², increasing the distance r from Earth's center reduces the gravitational field strength.
Physics Gravity and Gravitation 2
Kepler's Second Law (Law of Equal Areas) states that a planet: