A theorist has predicted a form of matter, called X-matter, that is gravitationally attracted to other X-matter but gravitationally repels ordinary matter. On a planet made of X-matter, an object made of ordinary matter would feel an "antigravitational" acceleration, pointing upward, away from the planet's surface. Such an object released near the ground would move not in free-fall motion but rather in "free-rise motion." Suppose that on this planet, a projectile launched downward from a cliff at an initial speed of \(20 \mathrm{~m} / \mathrm{s}\) is observed \(10 \mathrm{~s}\) later moving upward at \(40 \mathrm{~m} / \mathrm{s}\).

(a) What is the free-rise acceleration on this planet?

(b) At what instant after launch is the projectile's speed zero?