domingo, 27 de enero de 2019

Suspension bridges

For centuries, people have walked across bridges that are just a log that hangs from “cables” of twisted grass.And they look very risky! They are suspension bridges.

Suspension bridges, unlike beam and arch bridges, have roadways that are suspended (hung) from above. Almost all suspension bridges have supporting truss systems. These are located beneath the bridge’s deck (roadway or path). Truss systems are open networks of triangles. They allow wind to pass through. This helps stiffen the deck. It also reduces its tendency to sway and ripple.

Suspension bridges are the most expensive kind to build. They can also span far longer distances than any other kind of bridge.

domingo, 13 de enero de 2019

Why conserve energy?

 Because there are problems with every known source of energy.

Fossil fuels pollute and are in limited supply. Nuclear fission creates dangerous byproducts. Nuclear fusion still isn’t practical. Solar collectors take up a lot of space, wind farms aren’t very attractive, and building hydroelectric dams usually pushes people and animals out of their environments. So, it looks like energy conservation is the wise way to go.
Here are some ways to use less energy:
  • Many cities around the world now have bike-sharing programs. These let people rent bicycles on a short-term basis. Bikes can be picked up at one location and dropped off at another. Have you used one of these bikes?

  • In its time, the gasoline-powered engine was a miracle machine. It allowed people to get around more efficiently than ever before. But to cut down on pollution, people are searching for new ways to get around.
One alternative to gas-powered cars is hybrid cars. These run partly on gasoline and partly on electricity. Another option is all-electric cars, which are even more environmentally friendly. Electric cars don’t need gas. Even better, they emit no exhaust fumes. Some of them can travel more than 260 miles on a single battery charge. Or solar-powered cars may be other way; solar cars must be small and light, and while they don’t go very fast, they can go very far.

  • In Stockholm, Sweden, researchers are turning body heat into useful energy. A quarter-million people pass through the city’s train station each day. Machines in the ventilation system catch their extra body heat and use it to heat water. That water is then used to warm a nearby building. Energy costs in that building have been cut by 25 percent.

jueves, 20 de diciembre de 2018

Winter Solstice

On this day, the Sun hits its most southerly point. It’s right over the Tropic of Capricorn. This is the Northern Hemisphere’s shortest day and marks the start of winter.