Simple Machines

What is a Simple Machine?

• A simple machine is a device that helps to accomplish a task by redirecting or alleviating some of the work input required.

• Simple machines cannot create energy. They can only redistribute or produce force on an object.

• The six basic simple machines are the lever, the inclined plane, the wedge, the screw, the wheel, and the pulley.

• Simple machines help people lift or move heavy objects, split thick solids, and secure things together.

Levers• Lever: a long piece of wood,

metal, etc. that rests on a turning point (fulcrum) to lift or open something

• An object (load) is placed on one end, while a force pushes the other end down to move the object.

• Force applied x Distance (between force and fulcrum) = Load x Distance (between load and fulcrum)

• Examples: bottle openers, scissors, hammers (when pulling a nail out)

Lesser force (load)

Applied force

Lever Types• First Class Lever

– Fulcrum is between force input and load. – Input (effort) and load forces are on

opposite sides, but act in the same direction.

– The closer the fulcrum is to the load, the less effort is needed.

– Examples: see-saws, pliers, scissors

• Second Class Lever– Load is between force and fulcrum. – Effort and load forces are on the same side

of the fulcrum but act in opposite directions.

– The closer the load is to the fulcrum, the less effort is needed.

– Examples: staplers, nutcrackers

More Lever Types

• Third Class Lever – Force is between fulcrum and load.– Effort and load forces are on the same

side, but act in opposite directions. – Effort required to lift the load is greater

than the load.– Examples: fishing rods, tongs, brooms

• For first and second class levers: large output of force is gained over a small distance (force multiplier)

• For third class levers: small output of force is gained over a large distance (speed or distance multiplier)

Pulleys

• A pulley is a wheel and axle over which a rope or chain is pulled in order to lift or lower heavy objects.

• Pulleys change the direction of motion/force applied to lift an object.

• The amount of force required to move the object remains the same

• Using a pulley feels easier because you are working with gravity by pulling down on the rope, as opposed to working against it by pulling the object straight up.

< Without the use of a pulley, the required lifting force is 100lbs.

With the use of a pulley, the required lifting force is still 100lbs. >

Multiple Pulleys

• Force required when using more than one pulley to pull up an object is less than force required when using one pulley to pull up the same object.

• This is because the weight of an object is split by the number of pulleys you use.

• However, the distance of the rope multiplies by the number of pulleys you use also.

Two pulleys: force required is cut in half, rope length is increased.

Inclined Planes• An inclined plane is a surface set at an angle against the horizontal.• Inclined planes split gravitational force in two:

– Force parallel to plane– Force perpendicular to plane

• Only parallel force must be counteracted when moving objects up a plane• Force required to do work is less than when pushing on a flat surface• Mechanical advantage = slope/length of plane• Used as ramps or sloping roads, and can be combined to form a wedge

Wedges• Wedges consist of a pair of inclined planes set against each

other.• By moving the planes relative to each other, a wedge builds

up force in a direction perpendicular to the moving wedge. • Mechanical advantage = slope/thickness; The advantage

gained requires corresponding increase in distance• Wedges are used to separate or hold objects.• Door stops, axes, and teeth are examples of commonly used

wedges.

Wedge used to hold a door Wedge used to separate an object

Wheel and Axle• A wheel and axle is a simple machine

made up of two circular objects of different sizes rotating on the same axis.– The wheel has the larger diameter,

and turns about the smaller axle. • Because a wheel is basically a lever

that can turn 360o, effort or resistance force can be applied anywhere on that surface. The central point of the wheel and axle serves as the fulcrum.

• A force applied to the wheel is multiplied when it is transferred to the axle, which travels a shorter distance than the wheel.– Since the wheel is larger than the

axle, it always moves through a greater distance than the axle.

• The mechanical advantage (the number of times a machine multiplies the effort force) depends on the radius of the wheel and of the axle.

• The wheel allows objects to roll along the ground, decreasing surface friction by substituting rolling friction for sliding friction.

• The wheel can also be used to lift objects by wrapping a cord attached to a weight to the axle. It is less efficient than the lever at lifting objects.

• Examples: screwdrivers, doorknobs, windmills, gears, Ferris wheels

Use of the wheel and axle

Screws• A screw is another type of an inclined plane. It is a

helical inclined plane wrapped around a cylinder to form a spiral, with a wedge at its tip.

• The main use of a screw is to hold objects together with its helical grooves.

• A screw can convert a rotational force to a linear force and linear force to rotational force.

Use of Screws• The screw’s ratio of threading

determines its mechanical advantage. – Screws with wide grooves are harder to

turn, yet travel a shorter distance to their destination.

– Screws with narrow grooves are easy to turn, but have to be twisted over a greater spiral distance.

• One example of the screw as a simple machine is Archimedes’ Screw - used to bring water up from low sources to the location of use by passing through a helix.

• Other examples of screws include bottle caps, worm gears, drills, wrenches, jacks, and light bulbs.

Simple and Compound Machines

• A compound machine is composed of multiple simple machines working together to redirect or apply force.

• While a simple machine has only one motion, complex machines may have two or more.

• Examples of compound machines are a wheelbarrow, which uses a lever and a wheel and axle to redirect pushing force, and a bicycle, which redirects force from the user’s feet through pulleys to turn the wheels and propel the user forward.

Works Cited• http://www.uark.edu/depts/aeedhp/agscience/simpmach

.htm• http://www.enchantedlearning.com/physics/machines/Le

vers.shtml• http://www.simplemachines.info/netscape.htm• http://www.howstuffworks.com/pulley.htm?printable=1• http://www.sciencebyjones.com/simple_machines.htm• http://www.usoe.k12.ut.us/curr/science/sciber00/8th/

machines/sciber/intro.htm• http://www.mos.org/sln/Leonardo/InventorsToolbox.html