Month: July 2013

Balloon Zipline: Speed Measurements Using Video Analysis

2013-07-09 12.54.09

Last week, my students and myself built a balloon zipline. This turned out to be an awesome project that the kids had a blast with. However, being committing to tricking kids into doing math and science, I challenged the kids to measure the speed. Coaching them along, we were able to come up with some reasonable numbers in a few different ways.

Because we had such a good time with the project, I wanted to share the detailed breakdown of what we did and how we did it.

Building the Zipline:

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The line, strung across the room.

The concept is simple; string up a tensioned line, add a drinking straw as the carriage, and tape a balloon to it as the motor to propel it forward! For our build, I first attempted to use a super light cotton sewing thread. The string was too weak and flexiable and wouldn’t handle the motion.

I found a solution in using a length of some 22 gauge solid wire. Granted, most people don’t have a spare 30′ length to span a room, but any non-elastic cord should work perfectly. The trick is to keep it under as much tension as possible. The less wiggle room the zipline has, the faster we will travel!

Calculating Speed:

To calculate speed, we measured the time it took to pass a certain amount of the conveniently sized 1′ square tiles.

[latex]speed(ft/second)=frac{distance(feet)}{time(seconds)}[/latex]

From there, we determined the conversion formula to find miles per hour using dimensional analysis.

[latex]frac{Ft}{Sec}=(frac{1mi}{5208ft})*(frac{60sec}{1min})*(frac{60min}{1hr})=frac{miles}{hour}[/latex]

We used two different methods to measure the speed; by manually using stopwatches, and the more fancy ‘video analysis’ version.

Manual Measurements:

Using a team of three, one to release the balloon, one to start the timer and one to stop the timer, we measured the time between 10 of the 1′ tiles. Taking a couple of trials to get an average, the more the merrier!

Trial 1 1.25
Trial 2 1.26
Trial 3 1.72
Trial 4 1.33
Trial 5 2.07
Average 1.526
Ft/second 6.55307994757536
  4.46800905516502

These are our results, popped into a spreadsheet to make things quick and easy.

 Video Analysis

If you have ever watched Mythbusters, you will have seen slow motion footage of some projectile or object (usually Buster) moving past a black and white striped background. They don’t often go into detail about this, but the idea is that looking at the footage and using the striped background as reference, they can determine things like speed and acceleration.

And that is exactly what we did. Taking advantage of the convenient 1′ tiled floor, we laid down two makers, the start and end point. Then, using my standard issue cellphone, we recorded video of the balloon shooting past. Setting the camera phone to shoot at the highest frames per second, we were able to look frame by frame and find the time the balloon past the first marker, and past the second marker.

Start Time 27.027
End time 27.227
Time Difference 0.199999999999999
Ft/second 15.0000000000001
MPH 10.2272727272728

Conclusions

I was really happy with how this little project turned out. Everyone had a blast, and everyone was able to tricked into doing some math over their summer break. I was able to have 6-10 year old kids pay attention as I scribbled formulas on the whiteboard and begin to understand our measurement process. What more could you ask for?

In the future, if I were to run this lesson again, I would love to use a high frame rate camera, and higher resolution backdrop. With this, we could measure the acceleration (or deceleration) as well as the speed, even chart the data in position versus time. Then derive the velocity versus time charts and acceleration versus time.

Adding in a third method would be a great idea too, using a microcontroller with trip sensors to accurately measure time and eliminate that pesky human error. With three total measurement methods, having the students thing about the best methods and using them to think of how we can measure other everyday features in the most effective way.

There is plenty of fun ways to mix up this project, by using different balloons, regulating the inflation of the balloons, building an accurate release method, etc, we can develop more accurate results and start eating away at variables. A process that all students should become familiar with.

So get out there and start launching some balloons!