Simple machines
From Thermal-FluidsPedia
Machines are an important part of our daily existence; they vary in complexity from simple tools like screwdrivers and scissors, to more complex machines such as cranes and automobiles. Whether powered by engines or by people, machines make our tasks easier. A machine eases the load by changing either the magnitude or the direction of a force. Simple machines are used to reduce the amount of force required to do particular types of work, but the trade off is that you must apply this force over a greater distance. Others, such as bicycles, are considered complex (or compound) because they are made out of a number of simple machines. There are six types of simple machines (See Figures 1a-1f):
1. Inclined plane (any slanted surface used to raise a load from a lower level to a higher level). Work is carried out using less effort by moving a smaller force over greater distances. Examples include ramps for wheelchairs, ramps to load luggage onto a plane, and escalators. It is believed that Egyptian pyramids were built using similar ramps.
2. Wedge (essentially two back-to-back ramps or inclined planes turned on their sides). Wedges are used mainly to distribute force on both sides. It works by applying a smaller force over a larger area. Nearly all cutting machines (chisels, axes, scissors, and knives) use different variations of the wedges.
3. Screw (the circular version of the inclined plane). A screw is really an inclined surface wrapped around a cylinder or a cone. Some examples are ship propellers, corkscrews, meat grinders, ordinary screws, and jar lids.
4. Lever (a bar or a rigid object that rests and rotates about a point called a fulcrum). Levers make work easier by allowing a small force to be applied over a long distance, instead of a large force to act over a short load arm. A lever is characterized by a fulcrum or pivot, a force (effort) arm and a weight arm. Depending on the position where force and weight apply relative to the pivot, there are three kinds of levers. In a first class lever, the fulcrum is between the force and the weight (pliers, seesaw, and scissors). When the weight is between the fulcrum and the force, the lever is called a second class lever (nutcracker, and wheelbarrow). In third class levers the force is located between the fulcrum and the weight (baseball bat, shovel, tongs, and human arm).
5. Wheel and axle (a wheel or spoke attached to an axle or a lever). Here, a smaller force applied to the longer motion at the wheel results in a shorter more powerful motion at the axle. Examples are windmills, gears, doorknobs, and steering wheels.
6. Pulley (a chain, belt or rope wrapped around a wheel). A pulley is often used to lift a heavy weight with less force. A pulley makes work easier because it changes the direction of force. Instead of lifting up, you will pull down on the load using your body weight. Like other simple machines, the trade-off is the longer distance that the smaller force must travel at the end of the rope. Examples are the bicycle chain and the pulley used to move a curtain up and down.
Question: Which of the six simple machines reduce the amount of work needed to raise a load?
Answer: None of them. Machines do not change the amount of work required. They merely make the work easier.
Question: How can a small child lift a heavy load?
Answer: In all the examples shown in Figure 2, the force required to carry out the tasks is reduced by making the lever arm bigger. In fact, we can show that the product of: force in x lever arm = force out x load arm
References
(1) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005