Question

If Pluto orbits the Sun in an elliptical orbit. Choose the correct option which is true of Pluto's orbit?

• A. The gravitational force acting between Pluto and the Sun must point in the direction of Pluto's velocity
• B. The gravitational force between Pluto and the Sun is greater when Pluto is farthest from the Sun
• C. The speed of Pluto is greatest when Pluto is closest to the Sun
• D. The speed of Pluto is greatest when Pluto is farthest from the Sun
• E. The gravitational force between the Sun and Pluto is constant at all points in Pluto's orbit

Answer 1

Answer: (C)

The speed of Pluto is greatest when Pluto is closest to the Sun

When the planet is closest to the sun, the potential energy of the system, $-\frac{GMm}{R}$ is lowest. Hence from conservation of energy of the system, the kinetic energy of Pluto is maximum, and hence it has the greatest speed closes to the Sun.