Final answer:
The normal force does zero work on a block sliding down an inclined plane, as it acts perpendicular to the direction of motion. Friction and the component of weight parallel to the plane do perform work, as they have components in the direction of motion.
Step-by-step explanation:
The force that does zero work when a block slides down an inclined plane is the normal force.
Work is defined as the force applied to an object times the distance moved by that object in the direction of the force.
Since the normal force acts perpendicular to the motion of the block sliding down the plane, it does not do any work. This is in contrast to the component of the weight parallel to the incline and the force of friction, which do perform work as they have components in the direction of motion.
When a block is on an incline, the weight of the object can be resolved into two components: one perpendicular to the inclined plane and one parallel to it.
The perpendicular component is balanced by the normal force, and since this component does not cause motion down the plane, the normal force does no work.
On the other hand, friction acts opposite to the motion and does negative work, and the parallel component of the weight acts along the motion, doing positive work.