If the force moving an object points at least partially in the opposite direction of motion, the work done by that force is considered to be negative.

Here's why:

* Work is defined as the product of force and displacement in the direction of the force.

* When the force is in the opposite direction of motion, the displacement is in the opposite direction of the force.

* This results in a negative dot product between force and displacement, leading to negative work.

Examples:

* Friction: Friction always opposes motion, so the work done by friction is always negative.

* Braking a car: The braking force acts opposite to the car's motion, slowing it down. This is negative work.

Key takeaway: Negative work doesn't mean the force is "bad" or "undesirable." It simply means the force is taking energy away from the object, slowing it down or changing its direction.