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Mechanical advantage is a measure of the force amplification achieved by using a tool, mechanical device or machine system. The device trades off input forces against movement to obtain a desired amplification in the output force. The model for this is the law of the lever.
Three classes of levers The three classifications of levers with examples of the human body. Levers are classified by the relative positions of the fulcrum, effort and resistance (or load). It is common to call the input force "effort" and the output force "load" or "resistance".
If a mechanical system has no losses, then the input power must equal the output power. This provides a simple formula for the mechanical advantage of the system. Let the input power to a device be a force F A acting on a point that moves with velocity v A and the output power be a force F B acts on a point that moves with velocity v B.
A compound machine is a machine formed from a set of simple machines connected in series with the output force of one providing the input force to the next. For example, a bench vise consists of a lever (the vise's handle) in series with a screw, and a simple gear train consists of a number of gears (wheels and axles) connected in series.
The mechanical advantage MA of a screw is defined as the ratio of axial output force F out applied by the shaft on a load to the rotational force F in applied to the rim of the shaft to turn it. For a screw with no friction (also called an ideal screw ), from conservation of energy the work done on the screw by the input force turning it is ...
If the input force F A is applied to the edge of the wheel A and the force F B at the edge of the axle B is the output, then the ratio of the velocities of points A and B is given by a/b, so the ratio of the output force to the input force, or mechanical advantage, is given by = =.
Examples of force. The following list shows different orders of magnitude of force. Since weight under gravity is a force, several of these examples refer to the weight of various objects. Unless otherwise stated, these are weights under average Earth gravity at sea level.
where F A is a force acting on point A on the rigid lever beam, F B is a force acting on point B on the rigid lever beam and a and b are the respective distances from points A and B to the pivot point. If F B is the output force and F A is the input force, then mechanical advantage MA is given by the ratio of output force to input force.