* Forces: Objects in motion are constantly affected by forces like gravity, friction, air resistance, and others. These forces cause acceleration or deceleration, meaning the object's speed changes.

* Real-world conditions: Even simple movements like rolling a ball on the ground involve changes in speed. The ball will slow down due to friction, or might speed up if pushed further.

* Constant speed is rare: For an object to travel at a constant speed, all forces acting on it must be perfectly balanced. This is a very specific and often temporary condition.

Examples:

* A car: A car accelerates from a stop, slows down for traffic, and might speed up on the highway. It rarely travels at a constant speed.

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

* A river: While the water may appear to flow smoothly, its speed varies due to obstacles, bends, and the force of gravity.

Exceptions:

* Objects in a vacuum: In the absence of air resistance, a projectile might travel at a constant speed horizontally.

* Objects in a controlled environment: In a laboratory setting, a frictionless surface might allow an object to move at a constant speed.

In summary: Constant speed is an ideal condition that is difficult to achieve in the real world. Most objects experience changes in speed due to the forces acting on them.