A force in the opposite direction to motion causes deceleration, also known as negative acceleration. This means the object's velocity decreases over time. Here's why:

* Force and Motion: Force causes a change in an object's motion.

* Opposite Direction: When the force acts opposite to the direction of motion, it counteracts the object's existing velocity.

* Deceleration: This counteracting force results in a decrease in the object's speed.

Examples:

* Braking a Car: When you press the brake pedal, the force of friction between the brake pads and the rotors acts opposite to the car's motion, causing it to slow down.

* Air Resistance: As an object moves through the air, air resistance acts opposite to the object's motion, slowing it down.

* Friction: When an object slides across a surface, friction acts opposite to the direction of motion, causing the object to slow down.

Note: It's important to remember that deceleration doesn't always mean "stopping." It simply means the object is slowing down. The object could still be moving, just at a reduced speed.