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.
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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.
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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.
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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.
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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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A force pushes or pulls something in a particular direction. Isaac Newton was the first person to make a study of forces. He realized that when an object produces a force on something else, there is an equal and opposite reaction. For example, if you wear roller skates and push against a wall, exerting a force, you will be pushed away with just as much force as you are applying.
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The loudness of sounds is measured in units called decibels (dB).Ordinary speech has a sound intensity of about 20 decibels, while a noisy room full of people produces about 70 decibels. The quietest sound you can hear is about 20 decibels. Loud music at a rock concert reaches a level of about 125 decibels, while sounds of 140 decibels or more actually cause pain in the ear. This level of sound can produce permanent hearing damage if it continues, which is why people operating noisy machinery have to wear ear protectors.
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Sound waves travel very well through water, so they are used to detect submarines or even shoals of fish. A sonar device under a ship sends out sound waves that travel down through the water. The sound waves are reflected back from any solid object they reach, such as a submarine, shoals of fish or the seabed. The echoes are received by the ship and can be used to ‘draw’ an image of the object, and its location, on a screen.
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Thunder is produced when lightning heats up the surrounding air very rapidly, causing it to expand faster than the speed of sound. This produces a ‘sonic boom’ like the one caused by a supersonic aircraft. If you count the seconds between seeing a flash of lightning and hearing thunder and divide this number by 3, it will tell you how many kilometres away from the lightning you are. Sound travels at about 1220 km/h, while light from the lighting strike travels so fast that you see it almost instantaneously.
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