* Velocity tells you the speed and direction of an object's motion.

* Acceleration is the rate of change of velocity. This means it can change an object's speed, direction, or both.

When acceleration acts in the opposite direction to velocity:

* Speed decreases: The acceleration works against the object's current motion, causing it to slow down.

* Direction may change: If the acceleration is strong enough, it can eventually cause the object to change direction completely.

Examples:

* A car braking to a stop: The car's velocity is forward, but the brakes apply an acceleration in the opposite direction (backward).

* A ball thrown vertically upwards: After leaving your hand, the ball's velocity is upward, but gravity acts downwards, decelerating the ball until it momentarily stops at its highest point.

Key Point: The important thing to remember is that acceleration doesn't always mean something is speeding up. It simply indicates a change in velocity, which can involve slowing down.