Vuelvo a ser feliz…

Static electricity is made up of electrons that don’t move, unlike the current electricity discussed above. You can generate static electricity yourself:

• Run a comb through your hair on a dry day and afterward it will pick up bits of paper.

• Rub a balloon against your clothing and then it will “hang” on a wall.

• Wearing socks, but not shoes, drag your feet along a carpet, and then touch a metal object, such as a doorknob. What happens?

• To see how two electrified objects will repel each other, tie two balloons on a string. Hang them side by side and stroke both of them on a clothing or piece of fur. Watch how they move apart.

Bridges

Ever since humans found that roads would give them the ability to travel from one place to another more easily and more quickly, they have been faced with the problem of crossing streams, rivers, gullies, and canyons. So they invented bridges—structures that span such geographic obstacles and make the way easier and more direct. At first, two basic geo- • Visit a construction site and see the plans that

builders are following. Talk with the project engineer.

• Visit a city planning office and talk with an engi- neer or surveyor. Let the boys look through the surveyor’s manual and learn how to read a sur- veyor’s rod.

• Tour with a knowledgeable guide a house that is under construction.

• Visit an operating drawbridge, grain elevator, ship-loading facility, or other large industrial operation that uses large cranes or other lifting equipment.

• Visit the municipal waterworks, sewage treatment plant, or a television or radio station and talk with the engineers who work there.

• Visit an electric generating or transmitting plant. • Look at some local bridges. Discuss what kind

they are and how they were made.

metric forms were used for bridges—the arch and the triangle—and they were built of stone and wood. Today, engineers use steel plates, wire cable, angles, I-beams, H-beams, and concrete to build the bridges we see reaching across interstate highways, rivers, and canyons.

Bridge design and construction depend on the kind of obstruction and how wide it is, the load the bridge must carry, the kind of ground or rock found at the site, and the cost.

Demonstration Bridges. Use stiff cardboard or

thin wood and brass fasteners to put together the shapes shown. Have Webelos Scouts experiment to see which type is stronger.

Strength of a Hollow Tube. This demonstra-

tion shows why columns—which provide added strength—are used in engineering and the building of bridges. Begin by laying one brick on a plastic foam cup that is lying on its side. The cup collapses. Then place another cup on its brim and add bricks (two or three) until it is crushed. Finally, glue four cups together, rim-to-rim and bottom-to-bottom, with white glue and allow the glue to dry. Place bricks (usually four) on top until the structure crushes.

Strength of an Arch

Materials: Two strips of thin cardboard, small jar, about six marbles, six books that are all the same size

Place the books in two stacks and lay the two strips of cardboard across them to form a bridge. Press down on the bridge with your finger and notice how weak it is. Now remove one of the cardboard strips and curve it into an arch. Wedge the arch between the stacks of books and place the other strip across the arch between the books. Now test your bridge for

strength. Place the jar on the bridge and put mar- bles into it one by one. You’ll see that the arch will support a lot more weight than a simple bridge.

Strength of a Triangle. This demonstration shows

why the triangle is the basis for many bridges. Using heavy cardboard strips and paper fasteners, make a rectangle as shown. Demonstrate how easily it can be collapsed by pushing on a corner. Now construct a triangle. Does the triangle collapse too?

Stone Arch Bridge. Make this bridge with three

pieces of cardboard glued together. The sides are 1 inch high and 4 inches long. The arch is 1_⁄

2 by 13⁄4

inches. The floor and ends are one piece, with the ends scored and bent. The floor is 37_⁄

8 inches long.

Each end is at least 1 inch long and 11_⁄

2 inches wide.

Draw the outlines of stones with a pencil.

Steel Plate Girder Bridge. This is typical of the

reinforced steel plate bridges used for roads and rail- roads. The bridge floor and sides are made from a 5-by-9-inch piece of cardboard. The floor is 3 inches wide and 9 inches long, with the 1-inch sides formed

by bending the cardboard up. The steel angles that would be used to reinforce the sides are simulated by drawing squares on the sides. Abutments are made from six layers of 5_⁄

8-by-3-inch cardboard

glued together. Pencil dots for rivets, and pencil shading along the sides to give the bridge a three- dimensional effect.

Modern Steel Arch Bridge. Steel arch bridges are

used by railways to span deep canyons. When the canyon is rock, no concrete abutments are needed. Make this bridge with three pieces of cardboard. The deck is 3 inches wide and 20 inches long. Sides (or railings) are 1_⁄

2 inch high. Make the sides and deck

as one piece, and then score and bend. Lay out one side and one end for the bottom unit as one piece on cardboard. The sides are 4 by 20 inches; the ends are 3 by 31_⁄

2 inches. The arch opening is 11 inches long by

3 inches high. Lay out vertical steel beams on 1-inch centers and mark them with a black felt-tip pen.