Introduction: (Initial Observation)
Every year thousands of buildings and bridges around the world collapse because of a flaw in design and some miss calculations. Among them is The Tacoma Narrows Bridge that collapsed on November 7, 1940 only a few months after it opened to traffic.
Every structure such as a bridge and a building is made of materials and to calculate the final strength of that structure, it is very important to estimate precisely the strength of materials being used.
“How strong is a tooth pick?” is a physics and engineering project that gives us the opportunity to make a fundamental look at calculating the strength of material. During this project we will explore methods that are being used by thousands of testing and engineering companies around the world.
Information Gathering:
Find out about material strength and how it can be measured. Read books, magazines or ask professionals who might know in order to learn about the woods used in making toothpick. Keep track of where you got your information from.
Think about materials around you. Do they all have the same strength?
Some materials such as nylon strings have a very high tensile strength. In other words if you hold the two ends of the string and pull them away from each other, you will need a high force to break the string. Now ask yourself if you have a string made of cellulose, or cement or ceramic, will that have the same tensile strength?
Some materials have a high tensile strength and can resist high outward forces. Some other materials such as cement and ceramic have high compressive strength and can resist high inward pressures. Knowing the strength of materials is important in designing structures. Some parts of a structure are subject to tension and some other parts are subject to compression forces. In this project you will measure the strength of a widely available household item, a toothpick.
There are many physics and engineering books (and web sites) that discuss the subject of, “Material Strength”. Based on the type of material and application you may need the tensile strength, compressive strength, or bending strength. These three properties together are known as material strength. Manufacturers of steel, aluminum and many other construction material usually test their products and publish such information for their customers.
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Following are some web sites about bridges and specially Tooth pick Bridges.
More about Bridges
http://www.factmonster.com/ce6/sci/A0808901.html
This introductory site provides some basic information about bridges.
Other Introductory Bridge Information:
2) Basic Bridge Types from Matsuo Bridge
http://www.matsuo-bridge.co.jp/english/bridges/index.shtm
3) Bridge Basics: A Spotter’s Guide to Bridge Design http://pghbridges.com/basics.htm
4) Bridge Building: Art and Science http://www.brantacan.co.uk/bridges.htm
5) Bridge Building Information http://www.brantacan.co.uk/bridges.htm
6) Bridges from Idaho PTV’s Building Big http://www.idahoptv.org/buildingbig/bridges.html
7) Geometry of Bridge Construction http://www.faculty.fairfield.edu/jmac/rs/bridges.htm
Bridge Pros
http://bridgepros.com/
This comprehensive bridge-site is dedicated to the engineering, history and construction of bridges.
Not-To-Be-Missed Section:
2) Bridge Projects http://bridgepros.com/projects/index.html
How Bridges Work (Webpage 1 of 7) by M. Morrissey at How Stuff Works
http://www.howstuffworks.com/bridge.htm
This website looks at the three major types of bridges so that you can understand how each one works. The type of bridge used depends on various features of the obstacle.
Super Bridge from PBS NOVA Online
http://www.pbs.org/wgbh/nova/bridge/
How would you span a freeway? A canyon? A river? Or an ocean waterway? Learn about the four major types of bridges and then test your knowledge by matching the right bridge to the right location.
Related PBS Website:
2) All About Bridges from Building Big http://www.pbs.org/wgbh/buildingbig/bridge/index.html
Question/ Purpose:
The main question is, “How strong is a tooth pick?”. But we also need to explore possible methods to test, measure, or calculate such strength.
Identify Variables:
For this project and at this level we don’t need to identify any variables. We just need to calculate the strength of a tooth pick in any sample condition. If we ever need to test the strength of a tooth pick in different physical conditions, then we would identify variables such as moisture, temperature, and other things that may affect the strength of a tooth pick. But we don’t need it at this time.
Hypothesis:
Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.
This is a sample hypothesis:
Since tooth picks are wooden products and wood is being used as a popular construction material, it should have a high strength. Based on a wood strength table found on the Internet, compressive strength of tooth pick should be about 5000 psi (pounds per square inch).
Experiment Design:
Following are simple procedures for measuring tensile strength and compressive strength.
Tensile Strength:
To test the tensile strength strength of tooth pick we need to pull both ends of a tooth pick in opposite directions until the tooth pick breaks. The ends of a tooth pick do not have a good grip; so we need to attach handles to both ends.
Get a wood dowel or any similar wood molding and cut it in small pieces of about 1.5 inches each. These pieces will become handles for our tooth pick. In one end of each handle make a 0.5″ deep hole the same diameter as tooth pick.
On the other end, but on the side, make a hole about 1/8th of an inch that later will be used to hang this handle to a nail or a robe.
You can modify all sizes and holes based on the material that you have access to. What we mansion here are just examples.
Let the glue dry at least for 24 hours. Then you should be ready to do the pull test.
To do the test, you must use a strong nylon string, pass it through the hole on one handle and hang it from any fixture (we used a steel pipe as you can see in the picture). We next pass another nylon string through the hole in the other handle and hang a bucket or other container to it. Now, we start to fill the bucket with weights till the tooth pick breaks. The amount of weight that it takes to break the tooth pick is the tensile strength of the tooth pick.
It is good if you also do some calculations and convert the result to Pounds per square inch (psi).
In our test we added 72 pounds of weight until the glue came off and still the tooth pick was not broken. Apparently 24 hours is not enough for glue to become hard.
Compressive Strength:
To test the compressive strength we should push both ends of a tooth pick inward until it breaks. Tooth pick has sharp ends and does not stand vertically on itself. So to test it’s compressive strength we need to somehow secure it to stand vertically. One possible way is building a structure with four legs, each leg is one tooth pick. Then we enter the load on the structure (all four tooth picks together) and then divide the results by four.
To make such a cubic structure, first build two flat squares like the picture and then connect two square to each other to make a cube. It is good if you first connect the tooth picks to each other using the strings and knots. Then apply some glue to create a strong and permanent bounding. Wood glue (yellow or white) are good choices. We also tested with hot melt glue, but it is not good because of high flexibility.
When the structure is ready, we need to cut the sharp points and level our structure. The reason of cutting is that sharp points are also weak and can simply break. Our test result will be more accurate if we do the test with full diameter of the tooth pick.
Initial attempt showed that our structure has a tendency to collapse on the sides. Obviously hot melt glue that we used for joints are very flexible and are not good for a strong wooden structure. So to prevent sliding to the left or right, we also secured the upper board on one side (as shown in the picture) Then we put a bucket on the upper board and started to add weights until the tooth picks break.
Instead of a bucket and weights, we could also put our structure on a kitchen scale and start adding pressure to the upper board until the tooth pick columns break. In this way the scale will show the maximum pressure that broke the tooth picks. The problem in this method also is that if tooth pick columns are not fully vertical or joins are not very strong, the structure may collapse on the side.
Another method for Compressive Strength:
A good method for testing the compressive strength is building a device that can hold the tooth pick ends and push them inward in a sliding form.
To make the slide you can use some wood moldings and make a channel. The with and height of the channel should match the sliding piece that you will insert later. Secure the molding with wood glue and give them enough time to dry before your test. Make sure that there will be no extra glue left in the channel, since it can prevent smooth movement of the slide.
If you have access to any clamps, use them to keep your pieces together while the glue is being dried. Also do not try to do all at once. Glue the first side molding and wait a few hours and then glue the second one.
When you glue the second side molding, use your sliding piece or any other same size pieces as a spaces to make sure that the distances will be accurate.
Sliding pieces are two piece of wood molding that can be pushed toward each other inside the channel.
Make a small hole on each sliding piece that will hold the sharp ends of the tooth pick. Insert a tooth pick and place the sliding piece in the channel.
As you see we also covered the instrument with a piece of clear plastic for more safety. To apply force we placed the instrument on a kitchen scale so the lower slide could not move and started to push down the upper slide.
In two different tests, tooth pick broke after about 17 pounds force.
We can do some calculations and based on the diameter of a tooth pick we can convert this number to pounds per square inch (psi).
In about 14 pounds pressure the tooth pick is being bent. Maybe if we could prevent the tooth pick from bending, it could carry much more compressive force.
Materials and Equipment:
List of material may be extracted from the experiment section. Materials for this project may be found at home or purchased from local hardware stores and craft stores. Be creative and substitute the materials and change the design to simplify your work when necessary.
Results of Experiment (Observation):
Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.
Calculations:
To calculate the tensile strength you must divide the maximum strength by the cross section area of the sample. Tensile strength can be calculated based on grams per square millimeters or pound per square inches.
For example if your toothpicks are round with a cross section of 2 millimeters, then the cross section area will be 1 x 1 x 3.14 = 3.14 square millimeters. If the maximum strength of this toothpick is 27 kilograms, then the tensile strength is 27 / 3.14 = 8.6 kilograms per square millimeters. (8600 g/s2).
If you are making your measurements in inches and pounds, this is another example for you.
If the diameter of the toothpick is 1/16″, then the radius is 1/32″ and the cross section area is 0.003 square inches. If the maximum strength is 50 Lbs, then the tensile strength is 16666 pounds per square inches (16666 Lbs/si).
Summary of Results:
Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.
It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.
Conclusion:
Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.
Related Questions & Answers:
What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.
Possible Errors:
If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.
If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.
References:
1. Search the Internet for “Bridge Collapse” or “Building Collapse”.
