1.    For this science fair project, the independent variable is the environment in which the pieces of wood  are kept for 2 weeks – whether in rain water, acid water, salt water  or wet soil. The dependent variable is the average strength of the wood after 2 weeks. This is determined by applying weight to the planks and  noting the breaking point of the wood. The constants (control variables) are the size of the wooden planks used, the length of time they are exposed to the environment, and the type of wood used.

2.    The experiment is  begun by inspecting the dimensions of the  wooden planks and ensuring that all  20  are of the same size. The 20 planks are divided  into 5 groups of 4 planks each.  All the planks in each group are labeled with the group’s letter – either A, B, C, D or E.

3.    The 4 pails are marked as either B, C, D  or E. The pails are prepared as  follows:

a.    Pail B  is filled with rain water. The 4 planks labeled B are placed inside it.
b.    Pail C is filled with a mixture of water and vinegar,  using an appropriate concentration of vinegar such that pH 4.5 is obtained. This is used to simulate acid rain. The 4 planks labeled C are placed inside it.
c.    Pail D is filled with water and salt. 40g of salt is added to every liter of tap water. This is to simulate sea water. The 4 planks labeled D are placed inside it.
d.    Pail E is filled with soil. Water is added to the soil daily to keep it wet. The 4 planks labeled E are placed inside it.

4.    The  planks labeled A are not placed in  any  pail. They are left  in a dry place and protected from accidental exposure to moisture.

5.    The wooden planks are left in the respective environments mentioned above for 2 weeks. After that  they are taken out of the pails and left to dry for 3 days.

6.    The strength of the wooden planks is tested  using the  set-up shown in  Figure 1 below.  2 stacks of 8 bricks each are placed some distance apart, and the piece of wood to be tested is placed across the stacks of bricks. The weights  are  hung from the center of the  plank, as shown.  More weights are added, 0.5kg at a time, until the plank breaks. The average weight that  caused the plank to break in each group is calculated and recorded in the table given below.