Final answer:
The mechanical advantage of a machine is the output force divided by the input force. Simple machines like levers and pulleys change the force magnitude to make work easier.
Step-by-step explanation:
The mechanical advantage of a machine is the ratio of the output force to the input force. Hence, the correct completion for the sentence 'A machine's mechanical advantage is the output force — the input force' would be 'divided by', as mechanical advantage is calculated by dividing the output force by the input force. Simple machines such as levers, gears, pulleys, wedges, and screws are devices that can change the magnitude of the force applied, effectively making work easier by reducing the input force required to achieve a certain output force.