This set of Physics Multiple Choice Questions & Answers (MCQs) focuses on “Gravitation – Kepler’s Laws”
1. From Kepler’s law of orbit, we can infer that the sun is located _____ of the planet’s orbit.
a) at the centre
b) at one of the foci
c) at both foci
d) anywhere along the semi-minor axis
Explanation: According to Kepler’s law of orbit, every planet revolves around the sun in an elliptical orbit and the sun is at one of the foci.
2. Kepler’s laws of planetary motion replaced circular orbits with _____
a) elliptical orbits
b) parabolic orbits
c) conical orbits
d) hyperbolic orbits
Explanation: From the first law of Kepler’s laws of planetary motion, we can infer that the orbit of a planet is an ellipse with the sun at one of the foci.
3. Kepler’s laws of planetary motion were proposed only for _____
a) our sun
b) any star in our galaxy
c) any star in the universe
d) stars of other solar systems
Explanation: The Kepler’s laws of planetary motion were published by Johannes Kepler between 1609 and 1619. They are three scientific laws describing the motion of planets around the Sun.
4. What does Kepler’s law of period relate?
a) Time period and semi-minor axis
b) Time period and eccentricity
c) Time period and semi-major axis
d) Time period and area swept by the planet
Explanation: According to Kepler’s law of periods, the square of time period f revolution of a planet is directly proportional to the cube of the semi-major axis of the planet’s elliptical orbit.
5. In the figure shown, what is the point “A” called?
Explanation: The point closest to the sun is known as “perigee” or “perihelion”. The velocity of the planet is the greatest at this point, in accordance with Kepler’s law of areas.
6. In the figure shown, what is the point “B”, which is the farthest from the sun, called?
Explanation: The point closest to the sun is known as “apogee” or “aphelion”. The velocity of the planet is the least at this point, in accordance with Kepler’s law of areas.
7. What is the time taken by a planet to sweep an area of 2 million square km if the time taken by the same planet to cover an area of 1 million square km is 36 hours?
a) 18 hours
b) 36 hours
c) 72 hours
d) 144 hours
Explanation:According to Kepler’s law of area, a line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.
If it takes 36 hours to sweep an area of 1 million square km, it would take (36 + 36) hours to sweep an area of (1 + 1) million square km.
Therefore, the answer is 72 hours.
8. The velocity of a planet is constant throughout its elliptical trajectory in an orbit.
Explanation: From Kepler’s law of area, we know that a planet sweeps equal areas at equal intervals of time. Therefore, when the planet is closer to the sun it sweeps a lesser area for a given velocity than when it was farther to the sun for the same given velocity. Hence, a planet travels faster when it is closer to the sun.
9. The velocity of a planet is the greatest at perigee.
Explanation: From Kepler’s law of area, we know that a planet sweeps equal areas at equal intervals of time. Perigee is the closest point to the sun and hence, the velocity of the planet is greatest at the perigee because, only then, can the planet sweep an area equal to that when it was farther from the sun for the same interval of time.
10. What is the constant of proportionality in Kepler’s law of periods known as?
a) Universal gravitational constant
b) Escape velocity
c) There is no constant of proportionality
d) Cannot be determined
Explanation: There is no particular constant of proportionality for Kepler’s law of periods. The law of periods only relates the proportionality of the square of the time period of revolution of a planet to the cube of the semi-major axis of the orbit of the planet.
11. Kepler’s laws of planetary motion improved ______
a) the heliocentric theory
b) the geocentric theory
c) the big bang theory
d) the string theory
Explanation: Kepler’s laws of planetary motion improved the heliocentric theory of Nicolaus Copernicus, replacing its circular orbits with epicycles with elliptical orbits.
12. The elliptical orbits of planets were indicated by calculations of the orbit of which astronomical body?
c) Earth’s moon
Explanation: By observing the motion of Mars in the sky, Kepler inferred that the planets have elliptical orbits around the sun. Kepler discovered that a simple ellipse would clearly define the orbit of Mars and eliminate many complexities. It would also eliminate the need for epicycles.
13. If the eccentricities of the planetary orbits were taken as zero, then the sun is at the centre of the orbit.
Explanation: If the eccentricity is taken as zero, then the trajectory would become circular. So, the sun would be at the centre of the circle since both foci would also lie at the centre.
Sanfoundry Global Education & Learning Series – Physics – Class 11.
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