Measuring the fall

Post date: Sep 27, 2015 7:40:33 AM

You've probably seen maple seeds falling from trees (or at least picked them up and dropped them before) - especially with autumn around the corner. These seeds have one or two "blades" that cause the seed to spin like a helicopter and increase the time it takes to hit the floor, which allows the seeds to travel further from their parent (a neat adaptation).

In the classroom, we can study this further by making our own falling helicopters with paper. This allows us to controllably change things like mass, wing dimensions, shape etc.

One way to measure the fall is using motion tracking software: a free cross-platform version is tracker (physlets.org/tracker/). The program takes photos(/videos) and it works out distances(/speeds/acceleration) if you tell it where the object is and give it a scale to work from.

Here's just a quick demonstration of what tracker can tell us (see picture below). I've loaded a video of the falling copter into tracker. I've defined the vertical/horizontal using the pink crosshairs and also a scale (the blue meter stick). You then SHIFT-click on the object you're tracking and tracker will skip to the next frame where you SHIFT-click again to capture it's motion (red points).

The nice thing are the graphs tracker can generate which are below (you can also export the data for more detailed fitting & analysis). The top panel has the vertical position against time. From this the velocity & acceleration are calculated (middle and bottom, respectively).

From a physics point of view, it's great to see the following features:

• • in the middle panel, you can see the velocity downwards increasing until it reaches a terminal velocity. You can also see the velocity going to zero quickly when it hits the floor.

  • The acceleration is shown in the bottom panel, which goes to zero when it reaches its terminal velocity (which happens after ~0.3 seconds). This is because the force due to gravity (weight) and air resistance are balanced, so there are no more changes to the speed (this is Newton's first law).

  • The upwards force from it hitting the ground is also seen in the bottom panel, as we now have a positive upward acceleration.