A construction worker needs to lift a heavy metal beam using a lever but has a limited length of plank. Which adjustment would allow the worker to lift the beam with the least effort if the position of the fulcrum can be changed but the plank length remains constant?
Move the fulcrum closer to the beam
The correct answer is to move the fulcrum closer to the beam. A lever works on the principle of moments (torque). At balance, the turning effect of the effort equals the turning effect of the load:
Effort × Effort arm = Load × Load arm
The efficiency of the lever is captured by its Mechanical Advantage (MA):
MA = Effort arm ÷ Load arm
Here the plank length is fixed, but the fulcrum can be repositioned. The load arm is the distance from the fulcrum to the beam (load), and the effort arm is the distance from the fulcrum to the worker (effort). If the fulcrum is moved closer to the beam, the load arm becomes short while the effort arm becomes long. This raises the ratio of effort arm to load arm, so the mechanical advantage increases. A larger MA means the same load can be lifted with a smaller effort — exactly what the worker wants.
This is precisely why a crowbar or a pry bar is always positioned with its fulcrum as close as possible to the heavy object being lifted or prised open.
Why the other options are incorrect: Keeping the fulcrum fixed at any point offers no control over the ratio of the arms and will not minimise effort. Placing the fulcrum at the centre of the plank makes the effort arm and load arm equal, giving a mechanical advantage of only 1, so the effort needed equals the full weight of the load. Moving the fulcrum closer to the worker does the opposite of what is wanted — it shortens the effort arm and lengthens the load arm, reducing the mechanical advantage and demanding more effort. Only moving the fulcrum toward the beam minimises the effort.
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