* Speed is the rate at which an object changes its position.

* Acceleration is the rate at which an object changes its velocity. Velocity is a vector quantity that includes both speed and direction.

Scenario:

Imagine a ball thrown straight up into the air.

* At the very top of its trajectory: The ball momentarily stops moving (speed = 0). However, gravity is still acting on it, pulling it downward. This means the ball has an acceleration of 9.8 m/s² downwards even though its speed is 0.

Other Examples:

* A car at a red light: The car is stopped (speed = 0), but the driver presses the gas pedal, causing the car to accelerate.

* A roller coaster at the top of a hill: The roller coaster momentarily stops (speed = 0) before accelerating down the hill.

Key Point: Acceleration is about *change in velocity*, not just change in speed. A change in direction also constitutes a change in velocity, and therefore acceleration.