A machine is a tool that helps us do work. They are useful because they can make a physical job easier by changing the magnitude or the direction of the force exerted to do work. For example, have you ever tried to unscrew a nut, bolt, or screw from something with your bare hands and discovered that it was just too tight to loosen even if you had a good grip? Well, eventually you had to get the proper tool, such as a screwdriver or wrench, to unscrew it. The tool is an example of a simple machine, which is a mechanism that has few or no moving parts. Simple machines make work easier and have three basic functions:
- To change the direction of a force
- To change the magnitude of a force
- To change the speed resulting from a force
There are six types of simple machines:
- Inclined Plane
- Wheel and Axle
A lever is a rigid bar that pivots about one point and is used to move an object at a second point by a force applied at a third. The force needed to use the lever is equal to the force needed to overcome the load times the ratio of the length of the lever arm from the fulcrum (pivoting point) to the load end and the length of the lever arm from the fulcrum to the effort end.
Figure 1: Lever
There are three classes of levers representing variations in the relative locations of the fulcrum, the load, and the force:
- First Class: The fulcrum is in the middle, and the load and effort are on either side. Example: a see-saw
Figure 2: First Class Lever
- Second Class: The fulcrum is at the end, with the load in the middle. Example: a wheelbarrow
Figure 3: Second Class Lever
- Third Class: The fulcrum is again at the end, but the effort is in the middle. Example: a pair of tweezers.
Figure 4: Third Class Lever
An Inclined plane is a flat surface that is higher on one end than the other. A sloping surface, such as a ramp or stairs, is an inclined plane and can be used to alter the effort and distance involved in doing work, such as lifting a load. The trade-off is that an object must be moved a longer distance than if it was lifted straight up, but less force is needed.
You can use this machine to move an object to a lower or higher place. Inclined planes make moving things easier. You need less energy and force to move objects with an inclined plane.
Figure 5: Inclined Plane
A wedge is a triangular machine that is thick at one edge and tapered to a thin edge at the other for insertion in a narrow crevice. It is used for splitting, tightening, securing, raising, and levering. It functions by converting a force applied to its blunt end into forces perpendicular to its inclined surfaces. The mechanical advantage of a wedge is given by the ratio of the length of its slope to its width. Although a short wedge with a wide angle may do a job faster, it requires more force than a long wedge with a narrow angle.
Figure 6: Wedge
A screw is an inclined plane wrapped around a shaft, cylinder, or pole. By rotating the screw, the force is applied perpendicular to the inclined plane, thus translating a rotational force into a linear one. It is often used to hold materials together. The inclined plane allows the screw to move itself when rotated.
Figure 7: Screws
A wheel and axle is a modified first class lever that rotates in a circle around a center point, or fulcrum. The larger wheel (or outside) rotates around the smaller wheel (axle). Bicycle wheels, Ferris wheels, and gears are all examples of wheel-and-axle machines. Wheels can also have a solid shaft with the center core as the axle, as in a screwdriver or drill bit or the log in a log-rolling contest.
Figure 8: Wheel and Axle
A pulley is just a wheel and axle with a groove around the outside. It uses the principle of applying force over a longer distance, and also the tension in the rope or cable, to reduce the magnitude of the necessary force. Complex systems of pulleys can be used to greatly reduce the force that must be applied initially to move an object. A pulley needs a rope, chain, or belt around the groove to make it do work. Pulleys are used to change the direction of an applied force, transmit rotational motion, or realize a mechanical advantage in either a linear or rotational system of motion.
Figure 9: Pulley