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 *unless acted upon by a net force*.
* Constant Speed: If an object is moving at a constant speed, it means there's no acceleration. Acceleration is the change in velocity over time, and velocity includes both speed and direction.
* No Net Force: Since there's no acceleration, there's no net force acting on the object.
To stop an object, you *do* need a force, but that force is needed to overcome its inertia and cause a deceleration (a negative acceleration). The amount of force needed depends on:
* Mass of the object: A heavier object requires more force to stop.
* Speed of the object: A faster object requires more force to stop.
Think of it this way: A car moving at a constant speed on a straight, flat road doesn't need any force to keep moving. But to stop it, you need to apply the brakes, which creates a force to slow it down.
