* Forces: Objects are constantly interacting with forces like friction, gravity, air resistance, and even the force you apply to them. These forces cause changes in speed and direction.

* Real-world examples:

* A car driving down a road will speed up, slow down, and change direction due to traffic lights, turns, and hills.

* A ball thrown in the air slows down as it rises due to gravity and speeds up as it falls.

* A satellite in orbit experiences constant changes in speed due to Earth's gravity.

* Constant speed is an ideal: The concept of constant speed is often used in physics to simplify calculations. It's a theoretical situation where no forces are acting on an object, so it continues moving at the same speed and direction forever.

Key takeaway: In the real world, objects are rarely traveling at constant speed. They are usually influenced by forces that cause them to accelerate or decelerate.