* Friction: Almost everything experiences friction, a force that opposes motion. This slows things down.

* Gravity: Gravity pulls objects towards each other, often causing changes in speed.

* External Forces: Objects are often acted upon by forces other than friction or gravity, like wind, collisions, or propulsion systems. These forces cause acceleration or deceleration.

Examples:

* A car: A car accelerates from a stop, travels at a roughly constant speed for a while, then slows down to stop.

* A ball thrown in the air: The ball slows down as it goes up due to gravity, stops momentarily at the top, and then speeds up as it falls back down.

* A river: The speed of the river changes based on the slope of the riverbed and obstacles.

Exceptions:

* Objects in a vacuum: In a vacuum, where there's no air resistance, objects can travel at constant speed.

* Objects in space: Objects in space can travel at a relatively constant speed for long periods due to the lack of significant forces acting on them.

Important Note: Even in these exceptions, true constant speed is hard to achieve. Tiny fluctuations in gravitational forces, for example, can still affect an object's motion over time.