Thursday, July 7, 2016
Waterfalls
Waterfalls are commonly formed in the upper course of the river.[1] At these times the channel is often narrow and deep. When the river courses over resistant bedrock, erosion happens slowly, while downstream the erosion occurs more rapidly.[1][2] As the watercourse increases its velocity at the edge of the waterfall, it plucks material from the riverbed. Whirlpools created in the turbulence as well as sand and stones carried by the watercourse increase the erosion capacity.[1] This causes the waterfall to carve deeper into the bed and to recede upstream. Often over time, the waterfall will recede back to form a canyon or gorge downstream as it recedes upstream, and it will carve deeper into the ridge above it.[3] The rate of retreat for a waterfall can be as high as one and half meters per year.[1]
Waterfalls are commonly formed in the upper course of the river.[1] At these times the channel is often narrow and deep. When the river courses over resistant bedrock, erosion happens slowly, while downstream the erosion occurs more rapidly.[1][2] As the watercourse increases its velocity at the edge of the waterfall, it plucks material from the riverbed. Whirlpools created in the turbulence as well as sand and stones carried by the watercourse increase the erosion capacity.[1] This causes the waterfall to carve deeper into the bed and to recede upstream. Often over time, the waterfall will recede back to form a canyon or gorge downstream as it recedes upstream, and it will carve deeper into the ridge above it.[3] The rate of retreat for a waterfall can be as high as one and half meters per year.[1]
Most motor vehicles have internal combustion engines. They are fairly cheap, easy to maintain, reliable, safe and small. Since IC engines burn fuel, they have long ranges but pollute the environment. A related engine is the external combustion engine. An example of this are steam engines. Aside from fuel, steam engines also need water, making them impractical for some purposes. Steam engines also need time to warm up, whereas IC engines can usually run right after being started, although this is not recommended in cold conditions. Steam engines burning coal release sulfur into the air causing harmful acid rain.
There are over 1 billion bicycles in use worldwide.[22] According to 2002 estimates, there are around 590 million cars in service in the world and 205 million motorcycles.[23][24] The most popular vehicle model in history is the Chinese Flying Pigeon bicycle, with on the order of 500 million in service.[25] The most popular motor vehicle is the Honda Super Cub motorcycle, having passed 60 million units in 2008.[26][27] The top selling car in history is the Toyota Corolla, with at least 35 million produced.[28][29]
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