<em>Explanation:</em>
<em>Explanation:An object is in motion when its distance from another object is changing. Whether an object is moving or not depends on your point of view. ... A reference point is a place or object used for comparison to determine if something is in motion. An object is in motion if it changes position relative to a reference point.</em>
Answer:
One is tension force that acts on the string and the other one is gravitational force that is acting on the pen and is therefore, the one that pulls the pen towards the ground. Therefore, the pen's motion is caused by the unbalanced force acting on the pen.
Added on wood, gasoline, wind