Tuesday, May 3, 2011

Strength and Structures with Third Grade

Tomorrow we will be working with the third graders to introduce strength and structures.  The takeaway lessons we want them to get out of these experiments are:
  • The strength of a structure depends on its size and geometric shape as well as material composition
  • Structures react to loading by a force by lengthening, being compressed, or bending
  • If a load is too large, the structure may fail
The first station has them review the idea of balance on a beam.  This is a topic covered in first grade, but it never hurts to review!  In this exercise, a simply supported beam (i.e., a yard stick) will be used to allow the children to play with different loads and to see how to balance them.  If two equal loads are equal distance from the center support the beam is in balance.  If one load is moved, then the beam will be out of balance.  You can balance an unequal load by changing the distance from the support as shown below in the second figure.

There will be two yard sticks set up for experimentation at each station.  With 5 or 6 children at each station, that should give each child some hands-on time.  I will also put several blocks of the same size at the station so they can play with balancing different loads.

In the second station, the children will look at load deformation of the yardstick.  The yardstick will be simply supported at each end.  There will be several blocks at the station and the students will load the beam until it begins to deform.


As you can see from the above photos, with a small load there is very little, if any deformation of the yardstick.  However, with a large load, the beam begins to deform and you can see it sag in the middle.  The students will be given a pile of blocks so that they can load the beam and see how it changes with loading.

 As a second activity, we will have the students turn the yardstick on its side.  In this configuration, the beam is narrower, but many times thicker.  We will load the beam in this configuration, noting that you can place the same load on the beam as in the other configuration and in this configuration  it does not deform. 

In the third station, they will load a rubber band and see how it changes with loading.  First, the kids will measure the length of the rubber band.  Next, we will have them attach the an empty plastic bag supported by a paperclip, there is no real deformation.  As we load rocks into the bag, the rubber band begins to deform.  As more rocks are added, the rubber band stretches even further.  We will have them measure the load and the amount of deformation using a yardstick and a spring scale.  Finally, we will have them remove the load and see if the loading caused any permanent deformation of the rubberband by measuring its final length.

In the last station, we will investigate torsional strength.  Some materials can withstand a significant amount of torsional loading, whereas others cannot.  In this station, each child will be given a piece of chalk and asked to twist it until it breaks.  Then they will be given approximately the same sized wooden dowel.  They will be unable to break the wooden dowel in torsion.  Wood is stronger than chalk.  Next, they will be given a toothpick and asked to break it.  The toothpick could be easily broken even though it is made of wood.  However, it is very small compared to the dowel. 

The purpose of these exercises is to get the kids thinking about how size, geometry and material type can affect the strenth of a material under loading.  The third graders do study matter and material.  Below is the text associated with the third grade standards relating to matter:

  • infer that objects are made of one or more materials based on observations of the physical properties that are common to each individual object.
  • compare the physical properties of smaller pieces of a material to those physical properties of the entire material.
  • conclude that materials have their own set of physical properties that are observable.
  • explain that physical properties are observable characteristics that enable one to differentiate objects.
  • design an investigation to determine if the physical properties of a material will remain the same if the material is reduced in size.

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