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Angle-of-Impact Calculator

Run the Angle-of-Impact Calculator Fullscreen

About This MicroSim

When a blood drop strikes a surface at an angle, it leaves an elliptical stain — the steeper the angle, the rounder the stain; the shallower the angle, the more stretched and narrow it becomes. Bloodstain pattern analysts use this geometry to recover the angle of impact from a simple measurement: sin θ = Width / Length.

This MicroSim lets you measure a stain and apply that formula. Size the elliptical stain, read the width-to-length ratio, and calculate the angle θ — then watch the trajectory diagram redraw to match.

How to Use It

  1. Drag the blue handles on the stain (or use the Width and Length sliders) to set its dimensions in millimetres.
  2. Read the live ratio: sin θ = Width / Length.
  3. Compute the angle yourself, then press Calculate to check your answer and draw the trajectory with a protractor arc.
  4. Press New Stain for a fresh randomized stain to measure and solve.

What You Can Learn

  • Calculate the angle of impact from a bloodstain's width and length.
  • Explain why a near-90° impact gives a round stain and a shallow impact gives a long, narrow one.
  • Connect the algebraic formula to the physical geometry of the drop's path.

You can embed this MicroSim on your own web page with this iframe:

<iframe src="https://dmccreary.github.io/forensic-science/sims/angle-of-impact-calculator/main.html"
        width="100%" height="482" scrolling="no"></iframe>

Lesson Plan

Audience: High-school forensic science (grades 9–12) Time: 15–20 minutes Bloom level: Apply (L3) — calculate the angle.

Worked example. Set Width = 15 mm and Length = 30 mm. Have students compute sin θ = 15/30 = 0.5, so θ = 30°, before pressing Calculate to confirm.

Guided questions:

  • Keep the length fixed and increase the width. Does the calculated angle get larger or smaller? Why?
  • What stain shape corresponds to an impact angle near 90°?
  • Why can the width never be larger than the length?

Extension. Discuss how analysts combine many stains' impact angles with their positions to triangulate the area of origin of the blood source.

References

Specification

This MicroSim was generated from a specification in Chapter 7: Bloodstain Pattern Analysis.

Design note: the angle answer stays hidden ("?") until you press Calculate, so the student performs the sin θ = W/L computation before checking it. Stain dimensions are constrained to width ≤ length, since the ratio cannot exceed 1.