What factor is key to determining the efficiency of a simple machine?

The efficiency of a simple machine primarily hinges on the friction encountered during its operation. Friction is a force that opposes motion and can lead to energy loss in the form of heat. When a machine operates, the input work done to move the machine's parts or load is often greater than the useful work output due to this friction.

To understand efficiency, consider the formula for efficiency, which is the ratio of useful work output to work input, often expressed as a percentage. If a machine experiences significant friction, a portion of the input work is converted to heat rather than contributing to the intended mechanical work, thereby reducing the machine's efficiency. Consequently, minimizing friction—through design, lubrication, or the choice of materials—can greatly enhance a machine's efficiency. In essence, the less friction a machine experiences, the closer the output work gets to the input work, resulting in higher efficiency.