Blood Spatter Lab: Angle of Impact

Transcription

1 Blood Spatter Lab: Angle of Impact Materials: Simulated blood Samples, yardstick, metric ruler Objective: Determine how angle affects blood spatter and to apply this to a mock crime scenario. Background: Forensic scientists examine bloodstain patterns to help determine the events surrounding a crime scene involving blood spatter. The unique arrangement of the blood spatter can tell a story. Blood spatter is just a collection of blood spots that may be different sizes and shapes formed by a variety of methods. Blood will spatter when an outside force breaks the surface tension of a blood drop. In other words, blood will not spatter until it hits a surface. Blood spatter is typically in the shape of an ellipse (a closed plane curve like an oval or circle). Mathematically speaking, the width and length of an ellipse have a unique relationship. By figuring out the ratio of the two and using a trigonometric function, scientists can determine the angle of impact at which the blood spatter occurred. This can help investigators recreate a crime scene or confirm a suspect or victim's story. Look at the following example: The width of the blood spatter above is 16 mm and the length is 24 mm. To determine the ratio between the two, divide the width by the length. Convert the ratio to an angle measure using a table of trigonometric ratios (see Table 1) or by using the sine function on a scientific calculator. Steps to follow when using a trigonometric table: 1. Divide 16 by 24 (.6667). 2. Look at the chart and find the number.6667 under the Sine column. 3. The number falls between.6561 and.6691 on the table. It is closer to corresponds to the angle measure If you are going to calculate the angle using a calculator, follow the directions in your calculator manual for the sine function. 6. In conclusion, blood spatter that is 16 mm wide and 24 mm long hit the surface at approximately a 42 angle.

2 As a blood spatter expert you have been called upon to examine pictures of a crime scene in which a person was shot. In the pictures the blood spatter is in the shape of almost perfect circles. Here are a few facts from the accompanying police report. Bob was shot by Rob. Bob and Rob are the same height. Rob's testimony claims Bob attacked him. Rob fell to the floor during the struggle. As Bob walked toward him, Rob fired the gun in self- defense while he was still lying on the ground. Bob, having survived the shot, claims the two men were standing face to face when Rob fired the gun. Based on the shape of the blood spatter, who is telling the truth? Justify your answer. Follow the experiment procedures to help you determine who is telling the truth. Procedure: 1. Using a metric ruler, measure the length and width of blood drop sample #1 appearing on the Simulated Blood Drop Samples handout. Record the measurements in the appropriate row in the Angle of Impact Table (below). 2. Divide the width by the length to get a decimal answer (round to the nearest ten- thousandths) and record this in the appropriate ratio column. 3. Change this ratio into the corresponding angle measure by using the Trigonometric Table in Table 1 (above). Record this number in the appropriate column in Angle of Impact Table.

3 4. Repeat this procedure for the remaining blood spatter samples. 5. Look at the samples. Note the shapes of the blood spatter. The more circular the blood drop, the closer the angle of impact is to 90. The longer and thinner the blood drop, the closer it is to 10. Data & Analysis: Angle of Impact Table Sample Number Width (in mm) Length (in mm) Ratio (as a Decimal) Angle of Impact in Degrees

4 1. What are some possible sources of error in this lab? 2. Do you feel that your results are accurate and would be admissible to court? Why or why not? 3. What other evidence might be useful in this case? 4. Besides angle of impact, what other serology information might be obtained specifically from the blood and blood spatter results in this mock crime scenario? Conclusion: Map out the angles you found in the Angle of Impact Table using the graph below to help you answer the following conclusion questions. (Hint: Each of the red lines from 0 to 90 indicates a 10 degree change. So, from 0 to the first red line above it indicates an angle of 10. The next red line past that one indicates and angle of 20 from the 0 origin and so forth.) 5. Did Rob fire the gun from a standing position or from lying on the floor? Be sure to explain how you determined the angle at which the gun was fired when it hit Bob.

5 6. Who is telling the truth, Bob or Rob? What made you decide on this conclusion? Standing at Floor at 0

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