Final answer:
The gravitational force and tangential speed decrease as the distance between a planet and the Sun increases, due to Newton's law of universal gravitation and Kepler's second law. The mass of the planet does not change, and the area swept out in a day stays constant due to Kepler's second law.
Step-by-step explanation:
The quantities that decrease as the distance between a planet and the Sun increases are the gravitational force and the tangential speed. According to Newton's law of universal gravitation, the magnitude of the gravitational force decreases in proportion to the inverse square of the distance between the two objects. Therefore, if the distance between a planet and the Sun is doubled, the gravitational force on the planet would be a quarter of what it was before. Moreover, according to Kepler's second law, as a planet moves farther away from the Sun along its elliptical orbit, its tangential speed decreases to maintain the conservation of angular momentum.
In contrast, the mass of the planet remains constant regardless of the distance from the Sun. The area swept out by the planet in a day remains constant as per Kepler's second law, which states that a planet sweeps out equal areas in equal times, and the distance traveled in a day would vary depending on the speed of the planet along its orbital path, not directly due to its distance from the Sun.