Explanation:
The gravity gives the pen a constant force pointing to the center of the earth. And because of Newton's Second law: The acceleration of an object is directly proportional to the force it receives, inversely proportional to its mass, and the direction of acceleration is the same as the direction of the force.
So the pen has a tendency to going straight down on the ground. I don't think it maintain a constant speed but a constant acceleration. And the rate of the acceleration is approximate 9. 8m/s^2.
And as for its inertia, Newton's original statement in Principia mathematica was that any body should remain in uniform motion in a straight line or at rest until an external force forces it to change its motion. Bodies are too lazy to move if you don't force them. The moment you let go of the pen, the initial speed is zero. And it gains a acceleration of 9. 8m/s^2
