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TESTING PAPERS FOR STRENGTH

Have any of you ever gone fishing? Have you ever bought fish line, or heard people talking about fish line? They may tell a fish story about the time they had a great big bass on the hook (in fish stories, all the fish are very large, strong, and smart!), and he got away by breaking the line. As soon as he or she tells that the fish broke the line, they tell that they were using, for example, "35-pound test nylon." That phrase, which includes a number of pounds and the word, "test," really means that to break that line, it takes a certain number of pounds of pulling. The number of pounds of pulling it takes to break the line (or string) is called the "tensile strength" of the line (or string).

It is another kind of measuring, and it is another kind of grouping.

If you were going to climb a mountain you would have to buy rope that would hold your weight. If you were a 200-pound man, you certainly would not buy 35-pound test rope! You would need to have rope with a tensile strength far greater than that with which you would catch a fish.

There are other qualities of line, string, and rope that are required for certain activities. This means that the manufacturers made several different measurements and
groupings to get the qualities that are needed to perform certain tasks.

This writer has never climbed a mountain, and has only seen photos and television pictures of mountain climbers. It might be that rope with a tensile strength of twice your weight would be needed. If you plan to climb a mountain, catch a big fish, fly a kite or sail a sloop, consult an expert, and get the correct information. What I want is for you to have fun and learn good stuff; I don't want somebody to fall off Mt. Everest just because they had rope of inadequate tensile strength!

We can find out about tensile strength in different ways, and we can make our own tests, as well.

Let's cut paper into strips of 3/4 inches wide by 8 inches long. We can use some different types of paper, so as to compare their strengths. Make loops of the strips, and glue them so they will stay in loops. (We're not decorating the classroom, so don't put the loops through each other to make paper chains!)

To find out which kind of paper loop has the most strength, without reference to pounds of tensile strength, we will attach a rubber band to a ruler with a thumbtack. We put a paper clip onto the rubber band, and slip one of our loops through the paper clip. Hook a finger through the loop, keep your eye on the top of the paper clip where it is attached to the rubber band, and pull straight down on the paper loop. Notice where the top of the paper clip is (on the ruler) at the time that the loop breaks. You have to be quick! As soon as the loop breaks, the paper clip will snap back up the ruler, because the rubber band will contract.

Now do the same thing with the other kinds of paper loops, and compare the spots on the ruler where the paper clip was when the loops broke. We really won't know the tensile strength of the loops, but we will know which ones are made of stronger paper. (If the loops do not break, but just come apart where we glued them, we have tested the strength of the glue rather than the paper.)

We can do it all again and learn the tensile strength of the papers by putting the loop onto a hook or peg that is attached to the wall. Onto the bottoms of the loop, we can, in turn, hang balance-scale weights. The weight of the one we try before the weight that causes the loop to break tells us the tensile strength of the loop. If we do this with each type of paper we use, we will not only know which type is stronger, we will know the tensile strengths of all the types of paper.

This experiment can also be done with different widths and/or shapes of paper loops, to see if that makes a difference in the tensile strength.

What kinds of really important things are done for which it is necessary to know and have the correct tensile strength? How about bridges that are suspended by cables? The
construction engineers need to know how heavy the bridge will be, add to that the probable weight of the vehicles that will cross it, and then decide what types and sizes of cable will be needed to hold up the bridge.

That is undoubtedly more important than knowing the tensile strength of some paper loops, but knowing how to determine the tensile strength of paper loops teaches us at least one way of measuring strength.

--o0o--

We will now test paper in another way, to learn its compression strength. We'll roll some sheets of paper into a tube, and hold it together with tape. Let's make our tubes
approximately 1 1/2 inches in diameter. Stand the tube on a scale and, keeping your eye on the scale indicator, push down on the tube until it is crushed. The weight it registered just before the tube folded is the compression strength of that tube.

We can see whether other kinds of paper are stronger or weaker than the first paper we used, by the same method. It also might be interesting to make tubes with the same kinds of papers, and the same number of sheets of paper, only make fatter tubes.

Several kinds of paper supplies come rolled onto cardboard tubes, and we sometimes receive packages in the mail that are sent in mailing tubes. Let's bring some cardboard tubes from home and test their compression strength, both across the width of the tubes and along the length of the tubes.

If we have several tubes that are alike in size and weight (which means they probably are alike in strength), we can test the compression strength of one, then cut the other in half lengthwise, and see if the short tube is stronger or weaker than the long tube. We can discuss the results of our tests, and see if can deduce why one is stronger than the other.

Compression strength of solids is important for many reasons. One big reason lies in the building construction industry. Buildings are supported by beams of different sizes and materials, according to the amount of weight they will support. We've all seen office buildings, apartment buildings, hospitals, etc. in the process of being built.
The steel beams that hold up the structures are put up early in the process. They look like skeletons. They ARE skeletons!

When buildings are designed, the building engineer must know the weights of the different parts of the buildings, add the probable weights of things that will be put into the buildings, and determine what size and strength of beams will be necessary.

Think what would happen if human bones were little, soft things like fish-bones. They would crumple as we grew! On the other hand, people don't need elephant bones, because we never get that heavy.

Older apartment buildings will not allow people to have waterbeds. Why do you think that is?

 
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