Simple Machines

Machine  -  device that makes work easier

Simple machine  -   device that does work with only one movement

Effort force  -  force applied to the machine

Resistance force  -   force applied by the machine to overcome resistance due to gravity or friction

Ideal machine  -   machine where work input equals the output

Mechanical advantage  -   number of times a machine multiplies the effort force

Lever  -   bar that is free to pivot or turn about a fixed point

Fulcrum  -   fixed point of a lever

Effort arm  -   part of the lever on which the effort force is applied

Resistance arm  -   part of the lever that exerts the resistance force

Pulley  -  grooved wheel with a rope or chain running along the groove

Wheel and axle  -  simple machine consisting of two wheels of different sizes that rotate together, such as a doorknob or wheel-handled faucet

Inclined plane  -   sloping surface used to raise objects, such as a ramp

Wedge  -   inclined plane with one or two sloping sides

Screw – simple machine consisting of an inclined plane wrapped around a cylindrical post

Compound machines  -   combination of two or more simple machines

Efficiency  -   measure of how much of the work put into the machine is lost as thermal energy produced as a result of friction

Power  -   the rate at which is done 

Watt  -  one joule per second

 

    Background Information/Fundamental Issues:

 Simple machines are basic devices that modify mechanical energy and perform work with only one movement. The simplest of these are the inclined plane and lever. More complicated variations consist of the pulley, wheel-and-axle, wedge, and screw. Today simple machines continue to play an important technological role in society, just as they have for thousands of years.

 Work is equal to force multiplied by the distance over which the force acts. The same amount of work can be accomplished with a small force over a larger distance as for a large force over a small distance. Essentially, simple machines trade distance for force or force for distance, but always at the expense of some input energy that is lost to friction. A machine--like pliers--is a force multiplier if a large output force over a small distance is obtained. A machine is a distance and speed multiplier if a small output force over a large distance is obtained, for example, a broom.

 Inclined planes are force multipliers that enable large loads to be moved up ramps with smaller forces that would be required if the loads were lifted directly. Common examples of this include roads, loading ramps, and escalators. Using an inclined plane reduces the amount of work to be done by increasing the distance while reducing the force needed.  The longer the inclined plane is, the easier the work is.  The distance is made greater while the force needed is reduced.

 Inclined planes are also the basis of the screw and wedge. A wedge is an inclined plane with one or two sloping sides.  The wedge changes the direction of the effort to help cut through an object.  They are often used to split materials, like wood.  The blades of an axe or a knife are examples of wedges.  In this case, the screw and the wedge are what move, instead of the materials they are used with.  A screw is the shape of an inclined plane spiraled around a cylinder post.  The winding planes are called threads.  Friction makes screws effective for fastening things together. 

 A lever is a rigid bar that rotates about a pivot called a fulcrum, like a seesaw. Levers can multiply either force or speed, depending on the relative positions of the handle, fulcrum, and load. Common examples include brooms, pliers, and wheelbarrows. The principle of the lever is found in a variety of machines including the pulley and wheel and axle.  Levers are things like a crowbar, hammer, shovel, or anything that is used to pry something loose.  They are also used to lift heavy objects.  A load is moved by increasing the distance from the fulcrum and the force being applied.  Therefore, the force applied at the end of a lever and the fulcrum and load to be lifted are at the other end.  There are three classes of levers.  A class 1 lever has the fulcrum between the force and the load, like a seesaw.  A class 2 lever has the fulcrum at one end, the force at the other end, and the load in the middle, like a wheelbarrow.  A class 3 lever has the fulcrum at one end, the load at the other end, and the force in the middle, like a human forearm.

 A pulley is a circular lever that consists of a grooved wheel with a rope or chain running along the groove. Combined with a rope that is wrapped in the groove, pulleys may simply change the direction of motion, or they may also multiply force to overcome large loads. A wheel and axle is also a circular lever but differs from a pulley in that the wheel is fixed to the axle, and both force and speed can be multiplied.

Pulleys are primarily used to lift objects.  A common example of a pulley is on a flagpole.  There are different categorizations of pulleys.  A single fixed pulley changes the direction of the force applied to the end of the rope.  A moveable pulley is one that is attached to the load to be lifted and therefore moves with the load as the rope is pulled.  For example, they can be used to lift heavy objects from the bottom of a cargo ship to the deck.  A block and tackle pulley is commonly used to lift sails on sailing ships.  They house several pulleys side by side and hold the axle in place.  A chain hoist is a pulley system joined together by a closed loop of chain that is pulled by hand.  A fixed pulley does not multiply the effort force, but a moveable pulley does, creating a mechanical advantage greater than one.  The mechanical advantage can be calculated by counting the number of support ropes excluding the effort rope.  Examples are doorknobs, screwdrivers, gears, and chain sprockets.

 Another type of simple machine is the wheel and axle.  A wheel and axle consists of two wheels of different sizes that rotate together, with the effort force applied to the larger wheel.  The small wheel exerts the resistance force.  A doorknob is an example of a wheel and axle.  The axle can also be a shaft that runs through the center of two wheels.  A wheel and axle can be thought of as a class 1 lever, because the fulcrum is between the force and the load.  A wheel and axle is used to move objects across distances, like in a car.  Gears are also a type of wheel and axle.