A simple machine is a device that acts to increase or decrease the magnitude of applied force. There are six simple machines: inclined plane, lever, wheel and axle, pulley, wedge, and screw. Energy is conserved in the operation of simple machines and the total work done on a simple machine must equal the total work done by the machine (including work to overcome friction, etc.). Simple machines operate by allowing a trade-off between the magnitude of force and the distance over which that force is applied.
In physics, work is defined as the product of force multiplied by the distance over which the force acts. With regard to simple machines, because the total work done on a system must equal the work output (i.e., the work done by the system) the products of force and distance must always be equal for various force and distance configurations. If the magnitude of force applied increases, then the distance over which the force acts must decrease. Correspondingly, if the magnitude of the force decreases, then the distance over which the force acts must increase.
The inverse relationship between magnitude of force and the distance over which the force acts are further described in terms of the mechanical advantage of simple machines. A mechanical advantage states a factor of force increase (i.e., a machine with a mechanical advantage of two exerts twice the force initially applied). Quantitatively, the mechanical advantage is equal to the force exerted by the machine divided by the input or applied force. The actual mechanical advantage of any machine is always less than the maximum mechanical advantage because there is always some undesired energy transformation (e.g. heating due to friction).
Undesired energy transformations also reduce the efficiency of machines. The efficiency of a simple machine is based upon the ratio of work output to input (Percent Efficiency = (W/ W) x 100).
The inclined plane is the simplest example of the trade-off of force for distance. In building the pyramids, for example, the ancient Egyptians used ramps to move heavy blocks of stone to great heights. The net work done in raising the stones against gravity did not depend upon the path taken, by applying a lower force over a longer distance, the same amount of work was performed on the stone to lift it against gravity. In moving any mass against gravity, the work required is the product of mass, gravity, and the height to which the object is raised. By using an inclined plane (a ramp) it is possible to use a lower force over a longer distance to raise the body. This is possible because the inclined plane resolves the weight of the body into vertical and horizontal components. A lower lifting force is therefore required to overcome the vertical or gravitational component.
A lever is rigid body designed to rotate about a pivot point. A downward force applied to one end of the lever results in an upward force on the other end. The rotational torque at a pivot point is directly proportional to the product of the magnitude of force applied and the length of the lever arm (distance from the force application to the pivot point). For a lever of fixed length, variations in the placement of the pivot point result in varying lengths of lever arms and thus differential rotational torques about the pivot point. Depending on pivot point placement, a lever can multiply either the force or the distance over which the force is applied. Levers are divided into three classes, depending on the relative arrangement of the pivot point (i.e., the fulcrum), the output force (i.e., the load), and the applied or input force (i.e. the effort). A first class lever (e.g., a crowbar) places the fulcrum between the load and the effort and the load is displaced in the opposite direction to effort. In a second class lever (e.g., a bottle opener or wheelbarrow) the load is placed between the effort and the fulcrum. And in a third class lever (e.g., a fishing rod) the effort is placed between the fulcrum and the load. In both second and third class levers the load is displaced in the same direction as the applied force. Although both first and second class levers amplify applied force, the third class lever acts as a force reducer (i.e., the output force is lower than the applied force.
In a wheel and axle machine, a wheel is fused to an axle so that the angular displacement of one results in the same angular displacement for the other. Because of equal angular displacements, the application of force to the rim of the wheel results in a shorter length but more powerful displacement at the axle. Correspondingly, a force applied to the axle that results in angular displacement turns the wheels through a greater distance (with the same angular displacement). In a fundamental sense a wheel and axle operate on the same principle as the lever. A tangential force acts on the wheel, with the long lever arm equal to the radius of the cylindrical wheel, and the short lever arm equals the radius of the cylindrical axle.
A pulley is a force reversal machine (i.e., pulleys reverse the direction component of force). Pulleys in a system of pulleys (e.g., a block and tackle), however, permit the lifting of heavier loads with the same application of force. The force distance trade-off requires that the rope move a greater distance than the load.
A wedge converts unidirectional force into forces that act at right angles to the wedge. The application of force at right angles to the wedge results in a splitting motion.
A screw is a variation of an inclined plane in which the incline is wound around the core or shaft of the screw. Consequently, a screw increases the distance of force application by the application of force along the thread path in such a way that, in turning a screw, rotational force is converted to a linear force that moves the screw body forwards or backwards.
Simple machines form the basic components of more complex machines and all machines, no matter how complex, are composed of combinations of the six different types of simple machines.
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