Here's why:
* Inertia is the tendency of an object to resist changes in its motion. This means that an object at rest wants to stay at rest, and an object in motion wants to stay in motion at the same speed and in the same direction.
* Newton's First Law of Motion (sometimes called the Law of Inertia) states that an object at rest will stay at rest, and an object in motion will stay in motion at a constant velocity (constant speed and direction) unless acted upon by an unbalanced force.
Therefore, an object moving in a straight line will continue moving in that straight line unless a force acts upon it to change its direction or speed.
