Tires were made of leather for thousands of years, and of iron or steel for hundreds. Pneumatic tires were invented in the 1840s and again in the 1880s. They became commonplace early in the 20th century.
Tires are made of different types of rubber. Softer rubber is used in summer or when the tires need better traction, for example, in auto racing. Tires made of harder rubber are made for long lasting performance, like long-distance truck carriers. There are many different types of tires. They come in different sizes and have different tread patterns.
There are many different sizes of tires. On car and truck tires, they are marked with 3 numbers and might look like: 225/60R16. The first number is the width in millimeters of the tire at the widest point when it is mounted and inflated. The second number is the sidewall (side of the tire) height as a ratio or percentage of the width. The last number is the wheel diameter in inches.
Tire size: 225/60R16
Very large tires (for example, tires on big mining trucks) use different units. For example: 59/80R63. The first number is the width in inches of the tire. The second number is the sidewall height as a ratio of the width. The last number is the wheel diameter in inches.
Most tires today do not have a tube inside of them. There are grooves in the wheel that let the tire be popped into place and hold a lot of air pressure. If there is a crack in the tire, it will not be able to hold its air. Tire leaks are very common. The most common cause is a hole from a nail or screw. This can usually be fixed by patching the inside of the tire so that it can hold air again. If the hole is close to or in the side of the tire, it can not be fixed. This is because the sides of the tire flexes to support the weight of the vehicle. A patch will not be able to handle the stress.
Mud and Snow, (or M+S, or M&S), is a designation used by manufacturers for all-season and winter tires. Most are designed to provide improved performance under low temperature conditions, compared to summer tires. The tread compound is usually softer than that used in tires for summer conditions. This provides better grip on ice and snow, but wears more quickly at higher temperatures. Tires may have well above average numbers of sipes in the tread pattern to grip the ice. M&S relates to the percentage of tread void area. On four-wheel drive vehicles, M&S tires are often standard equipment.
Population
A population is the number of living people that live together in the same place. A city's population is the number of people living in that city. These people are called inhabitants or residents. The population includes all individuals that live in that certain area.The world population was estimated to have reached 7.5 billion in April 2017. Asia is the most populous continent, with its 4.3 billion inhabitants being 60% of the world population. The most populous country is China with 1.4 billion people.
Population density is the average number of people in a place. Urban areas such as big cities have a high population density. People there live close to each other. In areas with a low population density, people usually live far away from each other, such as in rural areas out in the countryside.
Usually population refers to the number of humans in a certain area. The maximum population that can be supported in an area is called the carrying capacity.
Global population is going up, but the population growth rate is declining all over the world. Growth in poor countries is faster than in rich ones; some rich countries have a population pyramid that is nearly square. Urbanization is also common, and urban areas usually have lower birth rates. In population growth, births exceed deaths. In the modern world this is due to reduction of infant deaths, control of infectious diseases, and improved agriculture so more people can eat.
The change in population from 2010 to 2015 was:
Human population control is the practice of altering the rate of growth of a human population. Concerns about overpopulation and its effects on poverty, environmental degradation, and political stability led to efforts to reduce population growth rates.
Snow
Snow is a form of ice. Snow forms when water in the atmosphere becomes frozen. Snow comes in all different shapes and sizes.
At the freezing point of water (0° Celsius, 32° Fahrenheit), snow melts and becomes liquid water. Sometimes, the snow will melt very fast and become water vapor. This is called sublimation. The opposite, where water vapor becomes snow, is called deposition.
Snow is used for some winter sport activities like skiing and sledding. Sometimes people make artificial snow so they can ski. People also commonly build things out of snow for fun.
Snow can also be dangerous, as it can lower visibility and make driving very difficult. When it snows, the snow will melt a little during the daytime and freeze again at night. This makes ice which can make driving conditions very treacherous. Snow plows are used to remove snow from roads to make driving easier and safer. Also, sand or salt may be added to the road to help tires grip the road. When salt is mixed with snow, the snow will melt more easily. This is because salt water has a lower melting point than fresh water (water without salt).
A blizzard is a dangerous type of a snowstorm. A blizzard produces strong winds that keep the snow in the air, thus reducing visibility. Sometimes it produce thunder snow, which is snow with lightning and thunder.
West
West is one of the 4 main directions on a compass. West is normally to the left on most maps. For example: (Germany is located to the west of Poland, which is itself located to the west of Lithuania). When “The West” is used in everyday speech: it usually refers to the nations of Europe (Especially from Croatia Westward) and its daughter states in the Americas and Australasia. The sun and moon also set in the west.
List of physics topics
Air resistance
Compression --Critical angle -- Colour filter -- Concave lens -- Convex lens
Diffraction -- Density (distance)--(displacement)
Electromagnetic spectrum -- Energy --
Fluid friction -- Focus -- Force sensor -- Frequency -- Friction --
Flotation
Gamma ray -- Gravity
Inertia -- Interference
Kinetic energy
Lamp-housing -- Laser pointer -- Lens -- Longitudinal wave -- Longitudinal wave model (Crova's disc) --
Moment of inertia
Normal -- Normal force -- Newton's laws of motion
Oscilloscope - Overhead projector
Plane mirror -- Potential energy -- Prism
Rarefaction -- Ray box -- Reflection -- Refraction -- Ripple tank --
relative density refleaction of light through prism
Signal generator -- Slinky -- Slit plate -- Spring -- String theory -- Sound -- Statistical mechanics
Tension -- Translucent screen -- Transverse wave -- Transverse wave model --
Vibration -- Velocity
wave -- Wave speed -- Waveform -- Wavefront -- Wavelength --
Young's double-slit experiment
President
A president is the leader of a country or a company or other group. A president is usually elected by the people in that group. Voting is one way to elect a president.
The President of the United States is elected by the electoral college. Some other countries choose a president this way. In some, the Parliament does it. Some countries have direct elections to choose a president. Many countries have a monarch instead of a president and some have neither.
Companies have presidents. They are elected by the people who own part of the company. In some companies, the people who are workers for the company elect (vote for) their company president.