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A screw driver can quite easily be used as either a lever or as a wheel and axle device.
The most usual use of a screw driver is as a wheel and axle device. When you twist a screw driver to drive a screw, you are using its handle as the wheel and the shaft of the screw driver as the axle. You don't get much mechanical advantage because your wheel is hardly wider than your axle.
It's not uncommon to use a screw driver as a lever either. If you stick the end in between two things and try to pry them apart that way, you're using the screw driver as a lever.
A lever works on the principle of using torque. The torque is defined as force multiplied by its distance from a fixed point called fulcrum. A torque tends to create a rotational movement around the fixed point or the fulcrum. Mechanical advantage is obtained in the lever by varying the distances of the effort and load from the fulcrum. The torque produced by the effort is same as the torque resisting the action of the effort, but as their distances from fulcrum are different the magnitude of effort and load are also different.
A screw driver act as a lever because the the relatively broader handle produces much more torque for the same force as compared to the narrower blade of the screw driver. In this way, the screw driver is designed to provide a mechanical advantage of lever. The mechanical advantage of screw drivers can be increased by providing larger arms to grip the screwdriver instead of typical handle.
In machinery the principle of levers is also used in mechanism of wheel and axle, so that a small force applied to the wheel with its larger diameter is used to transmit a much greater force through the axle with much smaller diameter. For example the steering wheel of automobiles is made large to make it easy to steer the car. The screwdriver can also be considered a wheel and axle mechanism, in which the handle of screwdriver acts as the wheel and the blade (including the shaft) acts as the axle.
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