What defines mechanical efficiency?

Mechanical efficiency is defined as the ratio of useful work output to total work input. This concept measures how effectively a machine converts the energy it receives into useful work. The higher the efficiency, the more output is produced from a given amount of input energy, meaning less energy is wasted in forms such as heat or friction.

When we consider the other options, they do not capture the essence of mechanical efficiency. The total weight lifted by the machine does not reflect the input of energy or the conversion process, merely indicating the machine's capacity. The speed at which a machine operates may relate to performance but doesn't address how well energy is transformed into work. Lastly, the amount of energy consumed by the motor, while relevant, does not compare the useful output against this input. Hence, option A accurately encapsulates the fundamental definition of mechanical efficiency.