Grade 8 - Mechanical Systems
Introduction
What is a system? A system is a structure that has parts that are connected and influence each other in some way. It can be physical, such as a car, or organizational, such as the health care system. The input and output of a system are like the start and the finish. In order for the system to work it needs to have components that take the input and produce the output. When designing a system, considerations must be taken to maximize the effectiveness, the efficiency and the safety of the system
Section 1: Levers and Inclined Planes
Levers
A lever is a simple machine consisting of a rigid barand a pivot point. The pivot is called a Fulcrum. The part of the bar on which a person applies an effort force is called the effort arm. The portion of the bar on which the lever produces an output force is called the resistance arm.
The positions of the fulcrum, effort force, and output force vary among levers. Based on these differences, there are three classes of levers.
First-Class Levers
In a first-class lever, the fulcrum is between the effort force and the output force. Therefore, a first class lever changes the direction of the effort force. The output force is greater than the effort force when the fulcrum is closer to the output force than to the effort force—that is, when the effort arm is longer than the resistance arm. The mechanical advantage can be calculated by dividing the distance the effort arm moves by the distance the resistance arm moves OR by dividing the length of the effort-arm by length of the resistance. A see-saw is an example of a first-class lever.
Second-Class Levers
In a second-class lever, the output force is between the effort force and the fulcrum. Second-class levers do not change the direction of the effort force. However, they produce a mechanical advantage because the effort arm is longer than the resistance arm. A wheelbarrow is an example of a second-class lever.
Third-class lever
In a third-class lever, the effort force is between the output force and the fulcrum. Like second-class levers, third-class levers do not change the direction of the effort force. But, unlike second-class levers, third-class levers always produce an output force that is less than the effort force. A fishing roda and the hand muscles are examples of third-class levers. A third-class lever multiplies the distance of the effort. A person would only need to move her hands a short distance to move the tip of the rod through a greater distance.
Inclined Planes
An inclined plane is a straight slanted surface that can multiply an effort force. It makes it easier to move a heavier load upward.
The mechanical advantage is equal to the output force divided by the effort force. Suppose two students use ramps to slide boxes weighing 300 newtons onto a stage. One student uses a steeper, shorter ramp and applies an effort force of 225 newtons. The other uses a shallower, longer ramp and applies an effort force of 135 newtons. The effort force of the steeper ramp is 225 newtons. Its mechanical advantage is 300 divided by 225, which equals 1.33. The effort force of the longer ramp is 135 newtons. Its mechanical advantage is 300 divided by 135, which equals 2.22. The longer ramp has the greater mechanical advantage. The mechanical advantage can also be calculated by dividing the length of the incline by the height.
Screw
A screw is another simple machine. The spiral ridges called threads move into an object as the head of the screw turns. The space between the threads is called the pitch. A screw’s mechanical advantage is calculated in a similar way to a ramp’s. If the distance around the head of a screw were 1.5 centimeters and its threads were 0.1 centimeters apart, its mechanical advantage would be 1.5 divided by 0.1, which equals 15.
Wedges
A wedge is an inclined plane that changes the direction of an applied effort force. A knife is a wedge. When you push down on a knife to cut food, the knife presses sideways against the food, pushing it apart. A wedge may be a single inclined plane or two inclined planes joined back-to-back. Wedges that are thin have greater mechanical advantages than those that are thick.
Section 2: The Wheel and Axle, Gears and Pulleys
Wheel and Axle
The wheel and axle is a type of a first-class lever simple machine. There is a wheel that applies the effort force and a smaller axle that produces the output force. The mechanical advantage of a wheel and axle is calculated by dividing the length of the effort arm by the length of the resistance arm. The effort arm is the radius of the wheel. The resistance arm is the radius of the axle. Since the effort arm can be quite large compared to the resistance arm, this machine can have a large mechanical advantage.
Examples of wheel and axle machines
Gears
Like the wheel and axle, use rotation, but they transfer the rotation to a different axis, the gears are not attached, they use teeth that mesh together. The teeth on gears are very specifically designed. Their shape is one that allows them to be in contact with each other for as long as possible, but yet they do not cause an obstacle for rotation. Gears can be used to increase force (increase torque, increase mechanical advantage) or increase speed. They can also be used to change the direction or type of the force.
The mechanical advantage of gears is more complicated to understand than other examples, as it involves looking at torque, which we will not cover in grade 8. Instead, we will look at velocity ratio. First, we need to define the gears involved. The gear that you turn is called the "driver." The gear that turns as a result of the driver turning is called the "follower." Velocity ratio tells you how much faster the follower will turn than the driver. It is calculated as follows:
Velocity Ratio = Diameter or Driver / Diameter of follower
Alternatively, because of the carefully calculated way in which the teeth of gears are designed, you can also calculate velocity ratio by counting how many teeth are on the gears:
Velocity Ratio = Number of Driver Teeth / Number of Follower Teeth
Pulley
Pulley is a grooved wheel that turns by the action of a rope in the groove. When the rope moves, the wheel turns. A pulley is also a type of lever, one in which the rope forms the arms and the wheel serves as the fulcrum.
A pulley may be either fixed or movable. A fixed pulley makes work easier by changing the direction of the effort force. It does not change the strength of the effort force itself.
The wheel of a movable pulley is attached to the object being lifted and moves with it. A single movable pulley multiplies the effort force by 2, so it has a mechanical advantage of 2. However, a single movable pulley does not change the direction of the effort.
A pulley system is made up of several pulleys acting together. Some pulley systems contain both fixed and movable pulleys. The addition of a fixed pulley enables the system to change the direction of the effort. The mechanical advantage of a pulley system can be expressed in terms of the distance it moves an object compared to the distance its rope must be pulled when the effort is applied. This can be done by by dividing the distance the effort rope moves by the distance the object moves. A simple way to measure the mechanical advantage of a pulley system is to count the number of rope strands pulled downward by the object being lifted. This number is the mechanical advantage of the system.
Section 3: Energy, Friction and Efficiency
Work and Energy
In Science, work is defined as the force to move an object through a distance. Based on this definition, holding a heavy box in the same position results in no work being done because the box did not move.
Work is equal to the force of a push or pull multiplied by the distance the object is moved. The force must act in the same direction as the motion. If the force is expressed in newtons and the distance is expressed in meters, the units for the work done are newtonmeters (Nm), also called joules (J).
Suppose you use a rope to lift a bucket filled with rocks up to a tree house that is 5 meters high. The weight of the bucket is 30 newtons. You can calculate the work done by using the formula:
Work = Force X Distance
Work = 30 N X 5 m
Work = 150 Nm also = 150 J
Energy is the ability to do work. Like work, energy is measured in joules. There are several forms of energy. For example, an object placed on an elevated position stores energy because of its position and the force of gravity. This energy is called Potential energy. An object that is in motion also carries energy called Kinetic energy. This is due to the obect's mass and speed.
Energy usually changes from one form to another. When an object is moving uphill, it slows down, which means it loses kinetic energy, but because its moving to a higher elevation, it gains potential energy. when the object begins to move downhill, it gains more kinetic energy and loses potential energy.
Thermal energy is the heat energy in an object.
Conservation of Energy
Energy cannot be created or destroyed, however, it can change from one form to another. All forms of energy have a source, a means of transfer, and a receiver. For example, in a flashlight the energy source is the potential energy in the battery. An electrical circuit enables the energy to be transferred to the bulb. The bulb is the receiver of this energy. It can then give off energy in the form of light and heat.
The following are examples of various forms of energy:
- Nuclear energy - radioacive material, the sun.
- Electrical energy - battery
- Light energy - lump, the sun, flashlight.
- Chemical energy - food
- Mechanical energy - Moving parts of a machine
- Sound energy - Speakers
- Thermal energy - hot water, the sun
Changing Forms of Energy
Several devices convert one form of energy to another.
A hair dryer converts electrical energy into thermal energy
A speaker converts electrical energy into sound energy
A microphone converts sound energy into electrical energy
A bulb converts electrical energy into light energy (and some thermal energy).
Light energy from the sun is converted into chemical energy by the green leaves of trees through photosynthesis. This chemical energy can be transferred from plants to animals through ingestion. Some parts of the animals will convert this chemical energy into mechanical energy (muscles), thermal energy (for homeostasis), electrical energy (neurons) and sound energy (mouth).
Friction
Friction is the force that opposes the motion of an object. It occurs when two or more objects come into contact. For example, in order to move a book across a table, you must pull on it with a force that is greater than the force of friction that is reventing the book from moving.
There are many types of friction. For example, the force between the surfaces of two solid objects which keeps the objects from moving is called static friction. The force that opposes the sliding of an object over a surface is called sliding friction. Rolling friction is the force that opposes the motion of a wheel turning along a surface.
Friction is necessary to maintain position and prevent objects from falling. For example, friction is necessary when you want to stop your bicycle or to turn a corner. It prevents the wheels from slipping.
Friction and drag force are similar because both forces oppose motion. However, different types of friction do not depend directly on the size, shape, or speed of a particular moving object. In contrast, all three of these factors do affect drag force. For example, a crumpled piece of paper falls faster than another piece of the same paper that is not crumpled. This occurs because of the way that air affects differently shaped objects.
Net force is the sum of all the forces that are acting on an object. When the net forces are equal in strength and opposite in direction, they are balanced forces. The motion of an object remains unchanged. Forces of unequal strength or forces that are not opposite in direction are called unbalanced forces.
Balanced force
Unbalanced force
Efficiency
Efficiency is a ratio of the work obtained from a system to the work that is put into the system. If something was 100% efficient, you could put a set value of work into it, and achieve that exact amount of work from it. However, this is never truly the case.
Example:
A battery in a ride on toy car has 3000 J of energy left. It takes a force of 6 N, once started, on average, to keep the car moving? If a park is 400 meters away, should the parents let the kid ride his car there?
Work = Force X Distance
Distance = W / F
D = 6000 / 6 = 500 meters
This means, the car can ride a distance of 500 before the battery runs out. But the park is 400 meters away, so the car will be able to take the kids to the park but will not be able to get them back home because a round trip is in fact 800meters long.
If the car above ended up driving for only 480 meters, Efficiency can be calculated as:
Efficiency = Work Obtained / Work put in
= FxD / 3000 = 6 X 480 / 3000 = 0.96 = 96%.
So the car is only 96% efficient.
Section 4: Force, Pressure and Area
Forces
A force is a push or pull acting upon an object as a result of its interaction with another object. A force constantly applied to an object is called continuous force. A rocket engine provides thrust, which is a strong push in the direction opposite an object’s weight. Thrust causes the rocket to accelerate upward, away from the launch pad. This thrust will continue to be applied as long as the rocket engine burns fuel.
Types of Forces
We have already defined continuous force as that which gets exerted on an object continuously.
Momentary force is the type of force that acts on an object for a very short period of time. It can also be called impact force.
Friction is a force that opposes the motion of an object. Friction occurs when two or more objects come into contact.
Drag force occurs when an object moves through any liquid or any gas, such as air. This force opposes the motion.
Section 5: Hydraulics and Pneumatics
Pascal's Law: The pressure in an enclosed fluid is uniform throughout.
For example, When a pipe is punctured, or if it is not sealed properly, the fluid will rush out of the pipe. This is because the fluid is under pressure, and so is pushing outwards on the pipe. With no pipe wall there to hold it back, it exits.
The leak would stop if the pressure inside the pipe was lowered to match the air pressure on the outside.
Pumps
A pump is a device that forces fluids into an area. By forcing the fluid into an area it creates pressure in the fluid. The water in your home is an example of this. Water is pumped through the city water lines, putting it under pressure. When your faucet is closed, the water is blocked from escaping. Once you open the faucet, the water is pushed out of the tap by the pump. As more and more faucets are opened, more water is released. This means the pump has to supply more water. If the pump can not keep up, you will lose "water pressure" i.e., the water will not come out as quickly, as there is not as much pressure in it.
Hydraulics
Fluids under pressure are not just used to supply things like water. We can use the force created by this pressure to make powerful tools.
Hydraulic System is a device that transmits a force through a liquid by using Pascal's law of constant pressure
Pneumatics
Sometimes gases are mor epractical to use.
Pneumatics is the study of pressure in gases.
A pneumatic system is a device that transmits a force by releasing a gas that is stored under pressure
Hydraulic systems use the force of a liquid in a confined space. Hydraulic systems apply two essential characteristic of fluids – their incompressibility and their ability to transmit pressure.
Pneumatic systems do not seal the gas (usually air) in the same way as hydraulic systems seal in the fluid it uses. The air usually passes through the pneumatic device under high pressure and then escapes outside the device. The high pressure air is used to do the work.
Examples of Pneumatic Systems
Staple guns and pneumatic nailers use pulses of air pressure to drive staples or nails into solid objects.
Sandblasters do exactly what the name implies. High pressure air blasts tiny sand particles out of a nozzle to remove dirt and paint from stone or rock.
Hovercrafts have a pump that draws air from outside and pumps it out through small holes in the bottom of the hovercraft.
Examples of Hydraulic Systems
A hydraulic system uses a liquid under pressure to move loads. It is able to increase the mechanical advantage of the levers in the machine. Modern construction projects use hydraulic equipment because the work can be done quicker and safer. There are many practical applications of hydraulic systems that perform tasks, making work much easier.
Earthmovers use hydraulics to move large amounts of dirt from place to place.
Hydraulics and Pneumatics in the Human Body
Life depends on the respiratory system, which is a pneumatic system in the body. The lungs that allow air to enter and leave the body as they contract and expand. Breathing depends on changes in air pressure resulting in intake or expulsion of air.
The body also depends on a complex hydraulic system – the circulatory system. The heart pumps the blood through blood vessels around the body, carrying food and nutrients to all cells.
Section 6: Combining Systems
A compound machine is a combination of two or more simple machines. For example, scissors include two levers and two wedges. The pivot point for the blades and handles is the fulcrum, and the blades are the wedges. A bicycle is also a compound machine. The pedals and wheel and axle machines. The gears are also wheel and axle machines. The brakes work as two levers.
The work put into a machine is always greater than its resulting work output because friction causes some of the work input to be lost usually as heat. The wasted energy reduces a machines efficiency. Efficiency is the ratio of the work done by a machine to the work that was put into it. To calculate efficiency, divide the output work by the effort work. Coating certain parts of a machine with substances such as oil can reduce friction thereby increase efficiency of a machine.
Section 7: Machines Throughout History
Prior to the industrial age, all products were hand made and every one was unique. In the early 1900's, however, a few individuals came up with the idea to standardize components so they may be interchangeable.
It is not just the automotive industry that has adapted automation and the assembly line as common practise. Now a days, most products are manufactured in this manner.
Famous machines that helped to change the world
Archimedes Screw. The ability to draw water uphill, against the flow of gravity revolutionised irrigation and the supply of water. Designed in 213 BCE by the polymath, Archimedes, the Archimedes screw is still used for irrigation today.
The printing press. In 1455, Johannes Gutenberg developed the first mechanised printing press. He converted an old wine press to enable a heavy screw to press a printing block against paper. His machine enabled a huge reduction in the cost of producing books and helped lead to a rise in literacy, knowledge and was a key part of the Enlightenment.
Calculator A very primitive form of the calculator was developed by Blaise Pascal. The first solid-state calculator was invented in the 1960s, with digital calculators using microprocessors being invented in the 1970s.
The Telescope. telescopeGalileo is credited with building the first telescope. This was later improved upon, and the first reflecting telescope was built in 1668 by Sir Isaac Newton. Newton used parabolic mirrors instead of lenses and which operated using reflection. His telescope designs would later be used in mapping the stars and gaining a much better understanding of the earth’s position in the Universe.
The Steam engine The first steam engine was built by Henry Newcomen in 1712, but this inefficient steam engine was limited to a stationary point, such as using in mines. James Watt played a key role in making the steam engine more efficient. The Steam Train. The first working steam engine is often credited to be Richard Trevithick in 1804. However, George Stephenson’s engines were more famous because of their greater impact. The Internal Combustion Engine The internal combustion engine enabled the development of the modern motor car and related transport.
Section 8: People and Machines
Science and technology have given us many different amazing machines that have made our daily tasks easier. The automobile caught on very quickly, but the ideal machine soon demonstrated its greatest flaw. Pollution of the environment was a result of more and more fossil fuels being burned, in larger vehicles. Improving machines brought lots of positives, but there were also some negative side effects (like pollution).
The Industrial Revolution
The invention of the steam engine transformed society. Simple machinery replaced hand labor since 1700. Water-driven spinning machines were used in 1769 and could the work of 12 workers. James Watt’s efficient steam engine and Henry Cort’s use of coal for fuel (instead of wood) to make iron started the Industrial Revolution.
The question of whether technology changes society or society changes technology is still a challenge today. The automobile uses cheap fuel and therefore more vehicles are being used. With cities so large, people need a vehicle to travel from place to place. OR, is the convenience of having a vehicle just societies’ reason to have larger cities? Because of the impact of scientific knowledge on society preferences for styles and sizes of vehicles changed. Larger vehicles polluted more and cost more to operate, so society wanted more compact fuel efficient vehicles. Today alternative fuel sources (solar-powered, electricity, hybrids, propane and hydrogen fuel cells) are being tested and are utilized to a very small extent.
Designing for Comfort
The testing systems that designers use provide scientific information to researchers, allowing them to decide what type of modification is best for its designed purpose. Comfort is an important criterion that is evaluated. For example, the wheelchair has gone through many improvements over the years. These changes happened because of the research into ergonomic designs and pressure put on the designers by the consumer.