Which statements describe the motion of a planet according to Kepler's second law? Check all that apply.

A. The speed of a planet varies as it moves in its orbit.

B. The speed of a planet is constant as it moves in its orbit.

C. A planet travels the same distance along its orbit in equal units of time.

D. A planet sweeps out an equal area in equal units of time.

E. A planet's orbit is a circle centered on the Sun.

F. A planet's orbit is an ellipse with the Sun at one focus.

A and D First, review Kepler’s second law. "A line joining a planet and the Sun sweeps out equal areas during equal intervals of time." Now look at the available options to see if they match the facts. A. The speed of a planet varies as it moves in its orbit. This is implied since the distance from the planet to the sun varies and because the area sweep over constant time is constant, the speed HAS to change. So this is true. B. The speed of a planet is constant as it moves in its orbit. This is the exact opposite to option A above, and since A is true, this must be false. Don't mark it. C. A planet travels the same distance along its orbit in equal units of time. This is simply a restatement of option B above, and since B is wrong, so is this. D. A planet sweeps out an equal area in equal units of time. This is a simple restatement of the 2nd law, so it's obviously true. E. A planet’s orbit is a circle centered on the Sun. This is a contradiction of the 1st law and doesn't even say anything about how the planet moves. So it's false. F. A planet’s orbit is an ellipse with the Sun at one focus. This is a statement of Kepler's first law. And it doesn't actually mention how the planet moves. So although it's true, it doesn't apply to the 2nd law. Don't mark it.