My Science School

               As we use up natural resources such as coal and oil, there is a constant search for new sources of energy. The Sun gives out vast amounts of energy, of which only a tiny fraction reaches the Earth. If we could use just a small part of this energy it would fulfil all the world’s foreseeable needs for power, but so far it has not been possible to find efficient ways to collect solar energy. We harness the Sun’s energy with solar panels. This energy is then used to heat water.

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            Like rockets, jet engines propel an aeroplane with a stream of hot gases. Unlike rockets, which produce hot gas as a result of burning an explosive mixture, jet engines burn fuel in the air that they draw in at the front of the engine?

            Air enters the front of the engine and is compressor wheel. Fuel is injected into the compressed air, burns fiercely and expands. The hot gas rushes out of the jet pipe at the rear of the engine. The hot gas passes through a turbine, which drives the compressor at the front of the engine.

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               As an aeroplane moves though the air, the air passes over the surface of the wings. These are shaped with a curved top surface and a flatter lower surface, which means that air passing over the top of the wing, has to travel a little faster than that below the wing. This lowers the pressure above the wing, while the air pressure below pushes up, and the end result is the lift that keeps the aeroplane in the air.

               The tail surfaces keep the wing at the proper angle to provide the right amount of lift. The power to propel the aeroplane can come from the engines or, in the case of gliders, from rising air currents.

 

               In 1884 an international conference decided that the 0 degree line of longitude, or meridian, would run through the Royal Observatory in Greenwich, England. As you move to the east from the Greenwich Meridian, the time is one hour ahead of Greenwich Mean Time for each degree you move. If you move to the west, time is one hour behind for each degree. Midday is the point where the Sun is highest in the sky, whichever time zone you are in.

               The amount of daylight varies with the seasons, so daylight - saving time or summer time, was introduced to make maximum use of daylight hours. In the northern hemisphere, clocks are reset one hour ahead in spring and one hour back in autumn. (In the southern hemisphere the seasons are reversed.

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               After the invention of sundials, other means of telling the time indoors were developed, such as hourglasses and burning candles. The invention of clocks, however, allowed far more accurate timekeeping. Early clocks were powered by a weight hanging from a fine chain (a pendulum), but later on springs were used to store energy. Most mechanical clocks and watches now contain a balance wheel that spins backwards and forwards, allowing an escapement wheel to move a very small amount each time it spins. This wheel is driven by the energy stored in the clock spring.

               Many watches and clocks are now powered by an electronic timer with no moving parts. It contains a tiny integrated circuit and a vibrating quartz crystal, which measures time with great accuracy.

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               The Earth spins on its own axis once every 24 hours, or day. The Earth orbits the Sun every 365  days, or year. Every 4 years we need to have a leap year with an extra day to make up the difference between the calendar year (365 days) and the time it takes for one complete orbit of the Sun. The year is divided into 12 months, which roughly correspond with the time it takes for the Moon to complete one full orbit of the Earth.

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               Sundials are the simplest way to measure time during the day. A simple stick casts a shadow that grows shorter until midday, when the Sun is at its highest in the sky. The shadow grows longer again after midday. A sundial simply casts its shadow onto a plate marked with the hours. Sundials have been used for many centuries, and are still in use today.

 

 

               People have always organized their lives by the passing of time. The earliest hunters had to hunt during the hours of daylight. When farming had developed, it was important for farmers to understand the seasons in order to plant their crops at the right time.

               Long ago, people realized that the movement of the Sun allowed them to recognize the time of day. They also realized that the movement of the Moon was regular and could be used to give measurements of roughly one month. Modern life is governed much more by time, and we now depend on highly accurate clocks to measure every second of the day.

 

               For thousands of years people have used levers as a way of transferring a force from one place to another, and to change the amount of movement that results.

               To move a heavy object, a long lever can help. The lever is free to move about a point called the fulcrum. The shorter end of the lever is placed under the object, and force is applied to the longer end. This will cause the object to be lifted, but the long end of the lever will have to be pushed down a long way to lift the object only a short distance. With a long-enough lever you could lift a car for a short distance.

 

               A gyroscope is a heavy metal wheel made with ball bearings that reduce friction. Once the wheel is set spinning at high speed in an upright position, the force of gravity will try to topple it. However, the effects of gravity are countered by another force called precession. This force means that a spinning body tends to move at right angles to any force that tries to change its direction of rotation. The effect is to keep the gyroscope in the same position until it has slowed down so much that gravity overcomes precession.

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               When you whirl something around your head on the end of a string it flies outwards, and appears to defy the force of gravity. Centrifugal force is acting to pull the object away. The object tries to fly off, and you are applying a force to retain it. This is called centripetal force. The centripetal force stops suddenly when you let go of the string, and centrifugal force makes the object fly away in a straight line. These forces are the reason why you can whirl a bucket of water around above your head without spilling a drop.

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               Due to a force called inertia, an object will remain still or travel in a straight line unless some force interferes with it. Friction is one such force. The greater the mass of an object, the greater its inertia, and the bigger the force that will be needed to start it moving. This is why it is very difficult to move a stationary car by pushing it, but much easier once the car starts to roll. A very strong man is able to pull along a railway carriage once it has started to move. Inertia is the explanation for the magician’s trick of pulling a tablecloth off a table without disturbing the china and cutlery on it.

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              Falling objects are subject to the force of gravity, and as they fall their speed increases, or accelerates, by 9.8 m per second every second. This increase continues until the friction of the air becomes so great that acceleration stops and the object then fall at a constant speed.

              Gravity produces the same force on any object, no matter how heavy or light it is. In a vacuum, a feather would fall and accelerate just as quickly as a brick. When a bullet is fired from a gun, the force of the explosion accelerates the bullet along the gun barrel. This force stops immediately the bullet leaves the gun, and friction begins to slow the bullet’s speed.

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               Friction is a type of force that stops things from moving when they have to slide past one another. It is the force that you have to overcome if you push something along. Friction also tends to slow down moving objects, which is why a bicycle will not travel very far if you stop pedalling. All forms of moving machinery are subject to friction. Oil is used to reduce the level of friction and this helps to make machines work more efficiently.

 

                Gravity is the force that pulls towards the centre of the Earth. No matter where you stand on the Earth’s surface, the ground is always ‘down’. The force of gravity depends on the mass, or amount of material, of an object — objects feel heavy because of their mass. Isaac Newton realized that gravity not only affects the Earth, but it also controls the movement of the planets and the stars, as well as the orbit of the Moon around the Earth.

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