Final answer:
Friction is the force that opposes motion and causes objects to decelerate, and it is considered a type of acceleration when its direction is opposite the object's velocity.
Step-by-step explanation:
Friction is a force that opposes motion and causes an object or system to slow down. It acts opposite to the direction of motion and can be thought of as a resistance to motion. For example, when a sliding box slows down on its own, it is because of the force of friction acting on it.
The force that is opposite of motion and slows objects down is known as friction. Friction is a resistive force that acts against the direction of an object's movement, causing it to decelerate. In a real-world context, without friction and other resistive forces like air resistance, an object would continue to move at a constant velocity due to inertia, as described by Newton's first law of motion. Even acceleration can be in the opposite direction of motion, leading to what we often refer to as deceleration, but in physics, this is still considered acceleration with a negative value.