6.
GRAVITY
Gravity is the force that makes things fall to the ground on Earth and holds the planets in their orbits (paths) around the Sun. The force of gravity acts over immense distances between objects in the Universe and holds them all together. The gravitational force between objects increases with their MASS. It also increases the closer they are. The gravity between objects on Earth is usually too small to notice.
CENTRE OF GRAVITY
On Earth, objects have a point, often near their centre, which is called their centre of gravity. The lower it is, the more stable they are. Cars are designed with their heavy engines near to the ground, to keep their centre of gravity low. This means they can corner at speed without tipping over.
FALLING FORCE
Which falls faster, a ball or a feather? In Earth’s atmosphere, the ball reaches the ground first because air resistance slows the feather down. In a vacuum, there is no air and therefore no air resistance. The feather and the pool ball fall at the same rate because gravity pulls them with exactly the same amount of force.
MASS
The mass of an object is the amount of matter it contains. The greater the mass of an object, the more matter it contains, and the more it pulls on other objects with the force of gravity. The mass of an object does not vary unless the amount of matter inside it changes for some reason. Mass is measured in kilograms (kg).
7.
RELATIVITY
Einstein realized that the speed of light is always the same. He then calculated that an object travelling near this speed acts strangely: it shrinks in length, increases in mass, and time slows down. He also calculated that mass alters space. So small objects do not travel in straight lines near a large object – instead they follow the distortions in space made by it. Centuries after gravity was identified as a force, Einstein’s theory of relativity explained why it works the way it does.
In traditional physics, gravity attracts one mass to another. This explains why a comet follows a curved path around the Sun. Einstein’s general theory of relativity explains gravity differently. Masses warp space and time a bit like heavy balls resting on a sheet of rubber. The bigger the mass, the more distortion, and the greater the pull of gravity. In 1921 Einstein was proved correct when the light from a star was shown to be bent by the warping effect of the Sun’s mass.
ACCURATE TIMEKEEPING
The effects of relativity are only detectable when things travel at very high speeds. To detect them, scientists need accurate clocks that use atoms to tell the time. Atoms of the element caesium vibrate at a precise rate. Atomic clocks measure time by counting these vibrations. Clocks such as the one above use a radio link to a central atomic clock to relay the precise time.
BIOGRAPHY: ALBERT EINSTEIN German, 1879–1955
When Albert Einstein was expelled from school, no one imagined he would become one of the most brilliant physicists of the 20th century. His theory of relativity was so strange that people refused to believe it at first. It was widely accepted only after he won the Nobel Prize for Physics in 1921.
7.
PRESSURE
When you press or push something, the force you apply is called pressure. Pressure is measured as the force you use divided by the area over which you use it. If you use a bigger force, or if you use the same force over a smaller area, you increase the pressure. We experience AIR PRESSURE all the time because of the weight of air pressing in on our bodies. WATER PRESSURE increases as you go deeper in the ocean.
AIR PRESSURE
The gases in Earth’s atmosphere are made up of tiny molecules that are constantly crashing into your body and trying to press it inwards. This pressing force is called air pressure. It is greatest at ground level where there are most air molecules. At greater heights above Earth, there are fewer air molecules and the air pressure is much less. It is possible to compress (squeeze) air, and this is used to inflate vehicle tyres and to power machines such as pneumatic drills.
AIR PRESSURE IN TYRES
Heavy construction machines have large tyres for two reasons. The compressed air in the tyre helps to absorb bumps, so the ride is much smoother than it would be with a solid wheel. Large tyres also help to spread the weight of the machine over a much bigger area. This reduces the pressure on the ground and stops the machine sinking into the mud.
WATER PRESSURE
Water behaves differently from air when it is under pressure. It cannot be compressed (squeezed). This makes it useful for transmitting force in machines, using a system called hydraulics. Water is also heavier than air, and an increase in water pressure affects humans more than a drop in air pressure. Even with a snorkel or other breathing apparatus, it feels much harder to breathe underwater. The water above you presses down from all sides on your body, so your lungs find it harder to expand to take in air. The deeper you go, the more water there is above you and the greater the pressure on your body.
Liquid pressure is used to carry force through pipes. The small force pushing down does not compress the liquid but moves through the liquid to push another piston a small distance upwards. The wider area of this piston increases the force applied.
CHANGING AIR AND WATER PRESSURE
The higher we go, the less air there is in the atmosphere above us. The deeper in the sea we go, the more water there is pressing down on us.
20,000 m (65,600 ft) HIGH
At this height, air pressure is less than one-tenth that at sea level.
AIRLINERS 11,000 m (36,000 ft)
Aircraft cabins are pressurized to allow us to breathe as easily as at sea level. Oxygen is also supplied in case of emergency, as there is less air at this height.
MOUNTAIN TOPS 7,500 m (24,600 ft)
At this height, climbers often use breathing apparatus to give them more oxygen.
SEA LEVEL
The human body is ideally adapted to deal with the air pressure at sea level.
120 m (400 ft) DEEP
Divers cannot go any deeper than this without special suits to protect them from the pressure of the water.
SUBMERSIBLES 6,500 m (21,300 ft)
Underwater craft such as submarines have strong, double-skinned hulls to withstand water pressure. The world’s deepest-diving crewed submersible can dive to 6,500 m (21,300 ft).
10,000 m (32,800 ft) DEEP
At this depth, the pressure of water is 1,000 times greater than it is at sea level.
8.
ENERGY
Scientists define energy as the ability to do work. Energy makes things happen. The energy in sunlight makes plants grow, the energy in food enables us to move and helps us to keep warm, and the energy in fuel powers engines. Energy comes in many different forms and can be converted from one form into another. The main types include POTENTIAL ENERGY , KINETIC ENERGY, and CHEMICAL ENERGY.
POTENTIAL ENERGY
Energy that is stored up ready to be used in the future is called potential energy, because it has the potential (or ability) to do something useful later on. An object usually has potential energy because a force has moved it to a different position or changed it in some other way. When an object releases its stored potential energy, this energy is converted into energy of a different form.
ELECTRICAL POTENTIAL ENERGY
When thunderclouds move through the sky, they build up a large amount of electricity inside themselves. This is known as static electricity, which is a store of energy. When a cloud builds up more static electricity than it can store, some of the electricity flows from the cloud to Earth in a bolt of lightning.
ELASTIC POTENTIAL ENERGY
This type of potential energy powers bows and catapults. It takes effort to stretch a piece of elastic or rubber because the forces between its molecules try to resist being pulled apart. As the elastic stretches, the molecules move away from one another and gain potential energy. The energy stored in stretched elastic can also be used to power such things as toy cars and model aeroplanes.
GRAVITATIONAL POTENTIAL ENERGY
A snowdrift on top of a mountain has a huge amount of potential energy. This is known as gravitational potential energy because it is gravity that is constantly trying to pull the snow down the mountain to the bottom. When an avalanche occurs, the snow gathers speed and its stored potential energy is turned into kinetic energy (the energy of movement).
KINETIC ENERGY
Moving objects have a type of energy called kinetic energy. The more kinetic energy something has, the faster it moves. When objects slow down, their kinetic energy is converted into another type of energy, such as heat or sound. Objects at rest have no kinetic energy. Kinetic energy is often produced when objects release their potential energy.
HAMMER STRIKING NAIL
A moving hammer has a lot of kinetic energy. As it strikes the nail, it slows down and loses its kinetic energy. The energy does not disappear, however. Some of it goes to split the wood to make way for the nail, some passes into the wood as heat energy, and some is converted into sound.
CHEMICAL ENERGY
This is the energy involved in chemical reactions, when elements join together into compounds. This energy is stored inside the compounds as chemical potential energy. The stored energy can be released by further chemical reactions. The food we eat stores energy that is released by digestion. Energy can also be released by burning the chemicals in a process called combustion. Fuels are chemical compounds that release heat energy by combustion.
FOOD AS CHEMICAL ENERGY
When humans or other animals eat food, they use its stored energy to keep warm, maintain and repair their bodies, and move about. Different types of food store different amounts of energy. The amount of energy a food contains is measured in kilocalories (called Calories for short).
9.
WORK
Scientists use the word work to describe the energy needed to do a task, by making a force move through a distance. The amount of work done is equal to the energy used and both are measured in JOULES (J). It takes energy to lift a weight a certain distance, because you have to do work against the force of gravity. POWERFUL machines can do lots of work in a short time. EFFICIENT machines waste relatively little energy when doing work.
EFFICIENCY
Efficiency is a measure of how much of its energy a machine converts into useful work. No machine ever converts all its energy into work: some energy is always wasted in the process. Car engines convert fuel into the energy they need in order to move, but get hot as they do so. This heat does not help the car to move, so a car is relatively inefficient, compared to other machines.
EFFICIENT MACHINE
Bicycles are efficient machines. They allow riders to convert muscle power into movement with little wasted energy. Racing cyclists wear aerodynamic clothing. Less energy is wasted overcoming air resistance, so more energy is used to move the bicycle.
POWER
Some machines can do work more quickly than others, and these are said to be more powerful. Power is the amount of work that something can do in a certain amount of time. Cars with bigger engines can go faster, which means they cover more distance in the same time. This means faster cars do work more quickly than slower cars, so they are more powerful machines.
JOULES
The amount of work done when a force acts over a distance equals the size of the force (measured in newtons) times the distance through which it moves (measured in metres). The work done is measured in joules, named after English physicist James Prescott Joule (1818–1889). An amount of work takes the same amount of energy to do it, so energy is also measured in joules.
ONE JOULE
One joule is the work that has to be done to make a force of one newton act over a distance of one metre. One joule of energy is needed to do one joule of work. It would take two joules of work to apply the same force for a distance of two metres.
HIGH-ENERGY FOODS
When a tennis player hits the ball, he does work. If he eats a banana before the match, his body can use the energy it contains to do this work. The energy value of food is measured in kilojoules or kilocalories (Calories for short). The body does not convert all the energy in food into useful work, so it is not 100% efficient.
10.
HEAT
Metal heated in a furnace shows that it is hot by glowing red and sending out sparks – but there is also some heat in ice and snow. Heat is the energy of movement, or kinetic energy, stored inside every object, hot and cold alike. Heat energy makes the particles (atoms and molecules) inside the object move about. TEMPERATURE is how hot or cold an object is, depending on its heat energy. Temperature is measured with a THERMOMETER.
MOLTEN METAL
When iron is heated in a furnace, it glows red-hot and then melts at a temperature of 1,535°C (2,795°F). At this temperature, its particles move about with lots of kinetic energy. At higher temperatures they move even faster, and the iron in the furnace starts bubbling.
ICEBERG
Ice is cold, but it still contains some heat energy. An iceberg is made up of particles of water, held in a rigid crystal structure. They still vibrate slightly. If the iceberg cooled down so that its particles stopped moving altogether, it would be at the lowest possible temperature that can ever, in theory, be reached. This is absolute zero.
TEMPERATURE
Temperature is a measure of how hot or cold something is. Things that have high temperature are hotter than things that have lower temperature, because they have more heat energy inside them. Any object can transfer heat energy to a colder object. As it does so, it cools down and its temperature falls. The colder object warms up and its temperature rises.
TEMPERATURE SCALES
Temperature is measured in degrees Celsius or Fahrenheit (°C or °F) or on the absolute temperature scale, in units called Kelvins (K). The Celsius (also called Centigrade) scale runs from freezing point (0°C) to boiling point (100°C).
THERMOMETER
This is a device that measures how hot or cold something is on a temperature scale. When things get hotter, their heat energy makes them expand or get bigger. This is how a thermometer measures temperature. As the liquid inside expands, it creeps up a tube, which is marked with a scale and numbers that show the temperature.
MERCURY IN BULB
The thermometer contains a small amount of liquid mercury in a glass bulb at the bottom. To take someone’s temperature, the glass bulb is placed inside their mouth. As the mercury is warmed by the person’s body, it expands up the tube, and climbs the temperature scale. A kink in the tube stops the mercury falling back too quickly, so the temperature can be read and recorded.
THERMOSTAT
A thermostat switches an air-conditioning unit on and off to keep a room at a constant temperature. As the room heats up, the brass strip inside the thermostat expands more than the iron strip attached to it. The strip bends inwards, completes an electrical circuit, and switches on the air-conditioning unit.