Part of Lesson Plan: Blood Spatter - Understanding the Clues

Activity Overview / Details

  Materials,

1) Eyedroppers.

2) Paper cups.

3) Newspaper.

4) A sheet of 8.5 X 11 paper (Graph paper if possible).

5) A liquid such as beet juice to simulate the blood.

6) A ruler.

7) A clipboard.

Set Up -Tell your students to do the following.

1. Spread newspaper out to help confine the mess and keep it off of the surface of the desk.
2. Put the 8.5" x11" sheet of paper onto a clipboard.  This will be your target site.
3. Pour a small amount of beet juice into paper cups to use as your reservoir.
4. Fill an eyedropper half way with beet juice.

Actions:

1) Now raise one end of the clipboard about 8" above the newspaper. Let the other edge rest on the newspaper. 

2) Hold the eyedropper above the graph paper about 12". Gently squeeze until one drop of juice leaves the eyedropper and lands on the graph paper on the clipboard. That is drop "A."

3) Now raise the edge of the clipboard until it is 10" above the desk. The other edge should continue to rest on the newspaper which is on the desk.

4) Raise the eyedropper until it is 12" above the clipboard centered over another part of the graph paper.

5) Squeeze another drop. That is drop "B."

6) Now lay the clipboard flat on the newspaper. Raise the dropper about 12" above the clipboard and let one more drop of juice land on the graph paper. That is drop "C." Make sure that it lands separate from the other 2 drops.

7) Now look at the three drops. One should be round and two should be elongated. One of the elongated drops should be longer than the other.

8) Set the paper to one side and let it dry.

9) When the paper is dry, use your ruler to measure the length and width of the three drops. Only measure the main body of the drops. Do not include the length of the rays.

10) Now, for each drop, divide the width by the length and record the ratio.

11) Look each ratio up in the chart, or use the Excel program and the formula =degrees (asin(0.n)) replacing the "n" with the ratio. The results will be the angle of impact of the juice striking the graph paper. (See attached Blood Spatter Calculation Chart.)

The same method that works for beet juice will also work for blood.

For an additional exercise use the first sheet of paper you dripped juice on. By applying the same method to those stains you will be able to determine the angle of impact that each drop of blood had. This will not, however, work for the blood pool area as in that section the stain will be too distorted.

In a crime scene we would select several drops as samples and measure their proportions and determine each the arc sine. Then, using a protractor set at each of the analyzed blood drops, we would get a visual idea of the angle of impact. We might use something like string and thumbtacks to attach the end of the string to the part of the wall each separate stain is on then stretch the string away at the angle of impact. If we do this with several drops of blood, where the strings intersect is the point of origin of the blood. This is where the victim stood, or, if the blood was cast off from a weapon such as a baseball bat, it will show you the path of each swing of the bat. 

• Blood Spatter Chart.doc [ Download] This is a reference item that students can use to find a solution to an angle of impact problem.