A rock is thrown straight up. At the top of the trajectory, the velocity is momentarily zero. Does this imply that the force acting on the object is zero?

No.

Explanation:

Due to the Universal Law of Gravitation two masses always attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.This is also true for the system rock-Earth, which means that at any point of its trajectory, the rock is attracted by the Earth with a force.Once thrown straight up, assuming that the air resistance is negligible, no other forces act upon the rock.So, even when at the highest point of its trajectory the rock momentarily is at rest, there exists a force acting on the object due to the gravity, which causes the rock to start falling down from there.