Here's why:
* Friction is a resisting force: It opposes the relative motion between two surfaces in contact.
* Opposing means going against: This means the friction force acts in the direction that would prevent the object from moving or slow it down if it's already moving.
Examples:
* Pushing a box across the floor: You push the box to the right, but friction acts to the left, making it harder to move.
* A car braking: The brakes create friction between the brake pads and the rotors, which acts in the opposite direction of the car's motion, causing it to slow down.
* A ball rolling down a hill: Friction between the ball and the ground acts uphill, opposing the ball's downward motion.
