Here's why:
* Inertia: Objects at rest tend to stay at rest, and objects in motion tend to stay in motion at a constant speed and direction. This is the concept of inertia.
* Force: A force is a push or pull that can *change* an object's motion. It can start an object moving, stop it, or change its direction.
Example:
Imagine a hockey puck sliding across the ice. Once it's in motion, it will continue to slide until a force acts on it. This force could be friction from the ice, a player hitting it, or a wall it runs into. In the absence of these forces, the puck would keep moving forever at the same speed and in the same direction.
Key takeaway: Forces are needed to *change* an object's motion, not to maintain it.
