My Science School

MACHINES

Simple machines make it easier for people to perform tasks, such as lifting or pulling, which would be difficult to do with muscle strength alone. There are six types of simple machines: the wheel, the screw, the lever, the pulley, the inclined plane, and the wedge. These machines change a force into a bigger or smaller force, or alter the direction in which a force acts. The most basic tools, such as crowbars or spades, are simple machines.

1. WHEEL (GEAR) Gears are toothed wheels that mesh and turn together, changing the strength, speed, or direction of a force. A force on the axle of a small gear driving a large gear will lead to a bigger turning force on the axle of the large gear.
2. SCREW The spiral thread on a screw changes a turning force into a much stronger up or down force. The screw has to be turned many times to create just a small up or down movement.
3. LEVER Most levers magnify the force applied to them, making it easier to move a load. A lever turns around a fixed point called a pivot. The further from the pivot the force is applied, the easier it is to move the load.
4. PULLEY A pulley is a rope looped around a wheel to make a load easier to lift or move. The more ropes and wheels are used, the less force is needed to lift the load, but the further the rope has to be pulled.
5. INCLINED PLANE This is a flat surface with ends at different heights. Moving an object up an inclined plane reduces the amount of force needed to lift it up, but increases the distance it has to travel.
6. WEDGE This triangular object is used as a blade to split something or inserted under an object to lift it. As a downward force is exerted on the wedge, its widening shape produces a sideways force on the object.
7. CART WHEELS These wheels allow the cart to move smoothly up the ramp. Unlike gear wheels, these are not classed as a machine, because they do not change the size of the force applied to them to help do something.

COMPOUND MACHINE A device that operates using a combination of simple machines, called a compound machine. Human force is applied only once - to turn the gear wheel. Each simple machine applies a force to the next machine until the tomato is sliced in two.

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DYNAMICS

Every object tends to resist any change in its speed or direction. This property is called inertia. An object’s motion is only changed when a force, such as a push or pull, is applied. A heavy, fast-moving object is described as having lots of momentum. The more momentum something has, the more difficult it is to stop. A moving object also possesses kinetic (movement) energy. The principles of dynamics, or how forces make things move, were explained by scientist Isaac Newton (1642-1727) in his three laws of motion.

NEWTON’S FIRST LAW The first law states that an object will stay still or continue to move at the same speed and in the same direction unless a force acts upon it. When cars approach each other in a crash-test laboratory, they move forward steadily. The dummies inside each car are carried along at the same speed as the car.

NEWTON’S SECOND LAW This states that when a force acts on an object, it makes the object change speed or direction. As the two cars collide, the front of each car exerts a force on the other car, slowing it down. The dummies inside are slowed down as they experience the force of the seatbelt.

NEWTON’S THIRD LAW The third law of motion says that when a force acts on an object, the object reacts by pulling or pushing back with equal force, but in the opposite direction. So, it is impossible for one car to push on the other without experiencing a push back with equal force.

1. Inertia If the dummy has no seatbelt, inertia will keep it moving forward at the same speed until it is stopped by a part of the car that has been slowed down by the impact - such as the windscreen.
2. Collision When two cars collide, kinetic energy is converted to other forms of energy, such as heat and sound, as the fronts of the cars crumple. This is called an inelastic collision.
3. Momentum If a heavy lorry collides with a car, the lorry’s greater momentum pushes the car along for some distance, but cars of equal weight and speed are halted, as their equal but opposite moment cancel out.

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MAGNETISM

Magnetism is an invisible force that attracts (draws towards itself) or repels (pushes away) certain materials. Any object that can attract or repel magnetic materials is classed as a magnet. The area around a magnet that is influenced by its magnetism is referred to as its magnetic field. It is strongest at the “poles” (usually the ends) of the magnet. Opposite magnetic poles will attract each other, while like magnetic poles repel each other. Planet Earth has its own magnetic field, driven by the molten material that circulates beneath the surface.

1. MAGNETIC MATERIALS When an unmagnetized magnetic material is placed in a magnetic field it becomes a magnet itself, either temporarily or permanently. Materials such as nickel and iron are easily magnetized and demagnetized and are known as soft magnets. Alloys (mixtures) of iron, nickel, and aluminium are difficult to demagnetize and are referred to as hard or permanent magnets.
2. ATTRACTION Iron filings sprinkled around a magnet will reveal the magnetic force field in action. If you bring two magnets together so that a north pole is facing a south pole, then the filings will bridge the gap, showing attraction.
3. REPULSION If you bring two magnets together with their two north poles or two south poles facing, you can feel the pushing force between them as their magnetic fields come into contact and the like poles repel each other.
4. MAGNETIC STRENGTH The strength of the attraction that holds all these objects together can also be used in industry. Large cranes with a lifting magnet are used to move tonnes of scrap metals and old cars, as well as to load heavy machine parts.
5. LODESTONE Nearly 3,000 years ago, people discovered that a strange type of rock could attract iron objects. This rock, called lodestone or magnetite, is a form of iron oxide with strong natural magnetism. The first compasses were made from lodestone.
6. MAGNETIC SCAN In a magnetic resonance imaging (MRI) scan, a patient is placed in a magnetic field and radio waves are passed through the body, causing molecules within body tissues to vibrate. Different tissues vibrate in different ways, allowing each part to be seen clearly.
7. MAGNETIC EARTH Electric currents circulating inside Earth as the planet rotates cause it to act like a giant magnet, with a magnetic field that extends thousands of kilometres into space. Earth has magnetic poles, which are near, but not the same as, the geographic North and South Poles.
8. COMPASS In use from around the 12th century, a compass contains a magnetic needle, which is free to rotate on a pivot. The compass needle will always align itself with Earth’s magnetic field, so that its needle points towards the magnetic north pole.

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ELECTRICITY

From running a home computer to lighting up the world’s cities at night, electricity powers almost everything that we use. Electricity is one of the most useful forms of energy because it can be readily converted into other types of energy such as light, heat, or movement. Electricity results from the behaviour of tiny particles, called electrons, each of which possesses an electric charge. When an electric charge builds up in one place it is called static electricity. If the charge flows from place to place it is known as current electricity.

• Electricity is generated when coils of wire are rotated in a magnetic field. This forces electrons along the wire to form an electric current.
• In power stations, the force to rotate the coils is provided by water power (hydroelectricity), or by steam heated by oil, coal, gas, or the process of nuclear fission.
• A current only flows if it has a circuit to travel around. A current needs a conductor to flow through, something to power, and, usually, an energy source to drive the current.
• All conductors have a certain resistance to the flow of an electric current. When a conductor resists the current, the electrical energy is turned into heat.
• Materials with electrons that cannot move are unable to conduct electricity and are known as insulators. Electric wire is insulated with plastic or rubber.
• Electricity leaves power stations through metal cables on tall pylons. The power is sent out at a much higher voltage than is used in most homes.
• Before arriving in a city, the voltage is reduced by a transformer at a sub-station. It then travels across the city in cables under the streets.
• In some earthquake-prone cities, such as Tokyo, Japan, electricity is carried in overhead cables. Underground cables would be too prone to earthquake damage.
• A flash of lightning is a giant spark of electricity driven by static electricity (charge) that builds up inside a cloud from a collision of ice crystals and water droplets.
• Static electricity also occurs when you comb your hair or take off a synthetic sweater. It is caused by two things rubbing together and creating a charge.
• The human body is full of electricity. The 100 billion nerve cells in each person’s brain work by sending messages in bursts of electricity (impulses).

1. North America The USA has less than 5 per cent of the world’s population, but uses more than 15 per cent of the world’s electricity, most of it generated in coal-fired power stations. In Canada, hydroelectric generation accounts for more than half of all electricity generated.
2. South America Some countries in this continent produce electricity using ethanol, a renewable “green” fuel made from the byproducts of sugar cane.
3. Antarctica The only people who live here are scientific researchers. Their electricity is mostly provided by diesel-powered generators, but also increasingly from wind and solar power.
4. Europe A variety of different sources are used to generate electricity in Europe, including nuclear power. France relies on nuclear power stations for more than three-quarters of its electricity.
5. Africa Less than half of the 1.3 billion people living in Africa have access to electricity, and in many countries people suffer from frequent power outages. Solar and geothermal generation holds great promise for the future.
6. Asia In recent years, China, Japan, and India have accounted for much of the increase in electricity generation and use. China relies on coal-fired power stations, but is rapidly increasing its use of solar power.
7. Australia Australia relies on coal-fired power stations for about half of its electricity, but in New Zealand 97% is generated by hydroelectric power.

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WATER

Water is a tasteless, odorless liquid. Although it appears to be colourless, in fact it is very pale blue. Each molecule of water is made up of two hydrogen atoms and one oxygen atom, giving it the chemical formula H2O. Water is Earth’s most common compound, found everywhere, from the oceans that cover 71 per cent of the planet to each cell of every living organism.

• Unlike most compounds, water can exist in all three states of matter, solid, liquid, and gas, within Earth’s normal range of temperatures.
• At sea level, water is liquid between 0 and 100°C (32-212°F), but below 0°C (32°F), it solidifies into ice, and above 100°C (212°F), it becomes gaseous water vapour.
• Unlike most other substances, water is denser when it is liquid than when it is solid - that is why ice floats on the top of water instead of sinking.
• When water freezes into ice, it expands by nine per cent of its volume with a force that can burst pipes and split rock.
• Earth is the only place in the Solar System where conditions allow water to exist in liquid form at the surface. Some liquid water exists under the surface of the moons of Jupiter and Saturn.
• Water is essential for life, so astronomers look for it when searching for life on other planets.
• The body of an average adult man contains more than 40 litres (70 pints) of water.
• You need about 2 litres (4 pints) of water every day to keep healthy.
• About 30 per cent of the world’s population do not have clean, safe running water at home.
• Water is not a resource that can be used up like oil. Water evaporates into the air, forms clouds, and falls back to Earth as rain. In areas of Earth that receive little rainfall, water can be a scarce resource.

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MATERIALS

Almost everything around us is made from some sort of material. Each has different properties, such as strength or flexibility, which makes it useful for making particular products. Some materials, such as wool or stone, grow or occur naturally. Synthetic materials are manufactured. Composite materials combine the properties of two or more materials to achieve the best possible product.

1. SILK: This natural fibre is made from the cocoon of the silkworm. Each cocoon may produce 3 km (2 miles) of silk. Silk can be woven into an extremely fine fabric, and is prized for its texture and its shiny appearance.
2. CONCRETE: Concrete is made by mixing sand, gravel, cement, and water. It is a liquid when freshly made, so it can be poured into a mould, where it sets to form an extremely hard and durable material.
3. KEVLAR: A light, flexible, synthetic material, Kevlar is used for protective clothing, such as bulletproof vests. Kevlar molecules are arranged in long chains with strong bonds between them, which makes Kevlar five times stronger than steel.
4. WOOL: This natural material comes from the fleece of sheep. The structure of its fibres means wool has a tendency to shrink, so it is often mixed with synthetic fibres to make easy-care fabrics.
5. CARBON COMPOSITES: These materials are strong and light and can be turned into complex shapes, such as sports equipment. They are made from the carbon byproducts of coal, oil, and natural gas.
6. METAL: When heated, metals can be shaped into anything from a paperclip to an aircraft. They are also good conductors of heat and suitable for carrying electricity.
7. PLASTIC: This group of synthetic materials is made from petrochemicals (derived from crude oil). They are strong, light, cheap to make, and can be shaped into flexible sheets, films, or fibres.
8. GLASS: This transparent ceramic is made by fusing sand, limestone, and soda at high temperatures, or by recycling old glass. Molten glass can be shaped in many ways, such as into windows, lenses, and threads for optical fibres.
9. LYCRA: Synthetic fabrics are designed to have better properties than natural materials, such as cotton. Lycra is a stretchy fabric that keeps its original shape, making it perfect for sports clothes.
10. RUBBER Natural rubber is an elastic material made from latex, a milky fluid from the rubber tree. Synthetic rubber is made from petrochemicals. Rubber is used in tyres and for waterproofing fabrics.
11. NYLON: Developed in 1938, nylon was the first synthetic fabric. It can be produced in extremely fine threads, is cheap to manufacture, and was first used as a replacement for silk in stockings and parachutes.
12. CERAMIC: Ceramic materials are made by heating different types of clay to a high temperature. China, bricks, tiles, cement, and glass are all ceramics. These materials are hard, brittle, and resistant to heat.
13. WOOD: Wood is a strong material compared to its weight, and is a good building material. It is also used for furniture and art objects because of its attractive texture. It is referred to as hardwood or softwood, depending on the type of tree it comes from.
14. COTTON: Cotton is a natural material produced from the long, flexible fibres in the fluffy seedpods of the cotton plant. The fabric is soft, comfortable to wear, and there is no static build-up as there is with some synthetic fabrics.
15. STONE: Stone is a natural material quarried from the earth. It is hard and heavy and can withstand great pressure. Stone may be cut using diamond saws or extremely high-pressure jet of water.

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GRAVITY

The force of gravity is a force of attraction that exists between all objects with mass, from microscopic atoms to stars and planets. On Earth, gravity can be seen when objects fall to the ground, pulled by an invisible force. In space, the force of gravity keeps the Moon in its orbit around Earth, keeps planets in orbit round the stars, and holds huge clusters of stars together as galaxies.

NEWTON’S DISCOVERY An apple falling from a tree is said to have inspired English scientist Isaac Newton (1642-1727) to explore the force of gravity. He developed a theory stating that every mass attracts every other mass by a force between both masses. The more massive an object is and the nearer it is, the greater its gravitational attraction.

BIRTH OF A STAR A star is born inside a cloud of dust and gas in space called a nebula. The dust and gas begin to clump together, forming a core. The clump’s gravitational attraction increases as its mass increases, dragging in more and more matter. The centre, or core, becomes so massive and dense that nuclear fusion begins, and the star begins to shine.

MOON AND TIDES As the Moon orbits Earth, its gravity tugs at the water in the oceans, making a mass of water bulge towards the Moon. The force of Earth’s spin creates a matching bulge on the other side of Earth. These bulges cause the regular rise and fall of the water level at the sea’s edge that we know as tides.

MASS AND WEIGHT These are not the same. An object’s mass is the amount of matter it contains. An object’s weight is the force exerted on its mass by gravity. This means that on the Moon, where gravity is just one-sixth of Earth’s gravity, an astronaut will weigh one-sixth as much as he weighs on Earth, although his mass is the same.

ZERO GRAVITY In orbit around Earth, astronauts become weightless and float about their spacecraft as if gravity did not exist. In fact, gravity is still pulling the astronauts and their spacecraft towards Earth, but as the spacecraft travels forwards, it is also continually “falling” as it follows the curve of Earth. The craft and astronauts are in a state of free-fall, but falling without ever reaching the ground.

CENTRE OF GRAVITY An object’s centre of gravity is the point at which it balances. An object with a low centre of gravity is more stable — so a sports car is more stable than a double-decker bus. The secret to driving a car on two wheels is to ensure that the centre of gravity remains above the wheels - any further over and the car will tip over.

EINSTEIN’S THEORY German-born scientist Albert Einstein (1879-1955) developed a theory of relativity to explain how gravity works in space. He compared space and time to a sheet of stretchy rubber, which everything in the Universe rests on. Massive objects like stars make a big dip in the rubber. Less massive objects like planets fall into these large dips and so are trapped orbiting stars. The dips create the effect we call gravity.

AIR RESISTANCE In a vacuum, gravity causes everything to fall at the same speed. However, if an apple and a feather are dropped from the same height in Earth’s atmosphere, the apple will fall faster. As they fall, objects are slowed down by air resistance, created by friction between the air and the object. The speed an object falls depends on the balance between gravity’s pull and the air’s resistance.

BLACK HOLES When a massive star dies, its core may collapse. As it shrinks, the core becomes ever denser and forms a region of space called a black hole. The force of gravity in a black hole is so strong that anything entering it is swallowed up, including light. Although invisible, black holes can be identified by the effect their gravity has on everything around them. Material being sucked into the hole heats up, emitting X-rays that can be detected by X-ray telescopes.

ACIDS AND BASES

An acid is a substance that produces positively charged particles of hydrogen, called hydrogen ions, when dissolved in water. The more hydrogen ions an acid contains, the stronger the acid is. A base is the chemical opposite of an acid. Bases produce negatively charged particles in water, called hydroxide ions. The more hydroxide ions a base contains, the stronger it is. Bases that dissolve in water are called alkalis.

• INDICATOR PAPER When a strip of indicator paper is dipped into a liquid, the paper changes colour. The colour can be compared to a pH scale to find out the acidity of the solution. pH stands for “potential of hydrogen”, and measures how many hydrogen ions the solution contains.
• CITRIC ACID The sharp taste in citrus fruit such as lemons and grapefruit is due to the citric acid they contain. Citric acid is often artificially added to manufactured foods and drinks to give a tangy sensation that tastes refreshing. Vinegar is made when bacteria convert the ethanol in alcohol into acetic acid.  
• VINEGAR The sour taste of vinegar comes from the acetic acid it contains. Every step on the pH scale is 10 times less acidic than the previous step, so acetic acid with a pH of 4 is 1,000 times less acidic than hydrochloric acid.
• HYDROCHLORIC ACID The lower the pH value, the stronger the acid. Hydrochloric acid, created when hydrogen chloride gas dissolves in water, has a pH of about 1. It is highly corrosive, capable of eating through metals.
• CHEMICAL HAZARD Strong acids and bases have to be stored in containers that will not be corroded by the chemical within. These containers are labeled with chemical hazard symbols that show the potential dangers.
• STINGER When a bee stings, it injects a mild acid into a person’s flesh, which causes a stinging sensation. Washing the sting with alkaline soap may relieve the pain by neutralizing the acid.
• LIQUID SOAP Soap is a weak base. It is made by combining a weak acid with a strong base, making it only mildly alkaline with a pH of about 8. An indicator paper dipped into liquid soap turns blue.
• LIMESTONE Calcium carbonate, or limestone, is a type of rock formed from the remains of Dead Sea creatures over millions of years. It is an important base, which is quarried and crushed to make fertilizers, paints, ceramics, and cement.
• WATER Pure water is neither acid nor alkali, but neutral, with a pH of 7. Rainwater is slightly acidic, with a pH of 5 to 6, while seawater is slightly alkaline, with a pH of between 8 and 9.
• CLEANING FLUID The strongest bases have a pH of 14 or more. Alkaline solutions with a high pH are used as cleaning materials as they dissolve fats. Cleaning fluids such as bleach and caustic soda have a pH of around 10.
• HYDRANGEAS The hydrangea shrub produces different coloured flowers depending on the acidity of the soil. On acid soils, it produces blue flowers, on alkaline soils, it produces pink or purple flowers, and on neutral soils, it has creamy white blooms.

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CHEMICAL REACTIONS

The atoms within a molecule are held together with links called chemical bonds. In a chemical reaction, the bonds between a molecule’s atoms break, and the atoms bond in a different way to form new molecules. In some reactions, elements combine to create a compound. In others, compounds break down into elements or simpler compounds. All the atoms from the original substance exist in the changed substance, but in different places.

1. REACTION When vinegar (acetic acid) and chalk (calcium carbonate) are mixed, a chemical reaction takes place. The acidic vinegar breaks down the chalk to release carbon and oxygen as bubbles of carbon dioxide. The starting materials in a chemical reaction are called reactants. The materials that exist after are called products.
2. DISPLACEMENT In a displacement reaction, the metal that forms part of a compound is removed and replaced by another metal. When a coil of copper is dipped into a clear solution of silver nitrate, the copper displaces the silver from the solution to form a blue solution of copper nitrate and needles of solid metal silver.
3. BURNING When the wick of a candle burns, it is reacting with oxygen in the air to produce ash and smoke. The burning also produces energy in the form of heat and light. In all reactions, energy is used up when bonds between atoms break, and energy is released as new bonds are made.
4. REACTION RATES The rate of a chemical reaction is affected by factors such as temperature, pressure, light, surface area, and concentration. It is possible to change the rate of a reaction by varying one of these factors. For example, increasing the concentration of dye in a solution will dye the material more quickly.
5. REVERSIBLE A few reactions are reversible. The molecules created by the reaction can be reformed into the original materials. The initial reaction is called the forward reaction and the reverse is the backward reaction. Dinitrogen tetraoxide breaks down into nitrogen dioxide when heated, but reverts when cooled.
6. APPLYING HEAT When a mixture of yellow powder sulfur and silver-grey iron filings is heated to a high temperature, a chemical reaction takes place and iron sulfide is formed. Without heat, the substances would not react with each other. Heat speeds up most reactions, and cold slows reactions down.
7. EXOTHERMIC Thermite is a mixture of aluminium and iron oxide. When it is ignited at a high temperature there is an explosion, as the chemical reaction produces a sudden release of energy in the form of light, heat, and noise. Reactions that produce heat are known as exothermic reactions.
8. OXIDATION Some chemical reactions happen around us naturally. One of the commonest reactions is oxidation - when substances gain oxygen. Oxidation is happening when metals rust, when wood burns, and when we breathe. In all these reactions, substances are reacting with oxygen from the air.
9. SOLUTIONS A solution is a mixture in which the molecules are mixed so evenly and completely that it seems like a single substance. However, in a solution, a chemical reaction has not taken place. Neither the solute (the substance being dissolved) nor the solvent (the substance that it is dissolved in) have changed.
10. CATALYST A catalyst is a molecule that helps bring about and speed up a chemical reaction, but does not change itself during the reaction. Natural catalysts are called enzymes. Bread dough rises because enzymes in yeast cause a reaction that produces bubbles of carbon dioxide when it is mixed with water and sugars.

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