Here's why:
* Inertia is the tendency of an object to resist changes in its motion. This means an object at rest wants to stay at rest, and an object in motion wants to stay in motion at the same speed and direction.
* Newton's First Law of Motion describes inertia: "An object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force."
So, if an object is moving in a straight line, it will continue moving in that straight line unless a force (like friction, gravity, or a push/pull) acts upon it to change its direction or speed.
