Science Buddies: "Ask an Expert"

HI,

I have done my experiment on blood spatter and forensics sciences by dropping simulated blood on a piece of paper at different speeds and i found that as the speed of the object/person increases the length of the droplets will increase whilst the width will decrease and have an inverse variation.

But i don't know why? is it to do with air resistance or the surface or angle of impact? gravity? I'm not sure

Thanks so much
Ruby

Hi rubygh,

The droplet pattern has to do with the horizontal velocity of the blood droplet. A blood drop with no horizontal velocity will be a circle (imagine pressing down a sphere into 2-D). However, if the blood drop has horizontal velocity, the bottom of the sphere and the top of the sphere hit the ground in different places, since it takes more time for the upper parts of the sphere to hit the ground. In this extra time, the sphere is still moving, hence the increased length. The decreased width is a result of conservation of volume. Given the same droplet, the width must decrease if the length increases. You can also think of this as a result of less blood hitting a specific spot. Imagine a fixed volume of water, being dropped in one big circle versus five small circles in a line.

Hope this helps,
IlI