November 8, 2014

I decided to take a second look at the speed of a dart coming out of a nerf gun.  In this experiment I used photogates to time how long it took the dart to cover a short distance.

Episode 12: Nerf Gun Dart Speed–part 2

When I analyzed nerf dart speed six months ago,  I  measured the dart launch using some high speed video.  You can find that episode here.  I came up with around 19 m/s.  This go round I used photogates to time how long it took the darts to cover the first 10 cm.  I then discuss how certain (or uncertain) I am about the measurement.   That is called uncertainty.  Even though it sounds weird, I am actually more “certain” or confident in my measurement in this experiment than I was before.  Partly  that is because I have a lot more data (30 trials shooting the darts) but because I checked it with a radar gun and against some high speed analysis my student did.

Check it out:

Here is my nerf gun:

Nerf N-Strike Maverick

Uncertainty is a topic I usually save for my AP Physics class, but it seemed like I should include a little in this episode.  It really is at the heart of every measurement we make.  Here is a site that does a pretty good job explaining the basics if you want to know more.

I was pleasantly surprised that my radar gun could track the dart and that it agreed nicely with the photogate data.  That added to my confidence in the photogate measurement.

A Student’s Data

One of my physics students (Derek H) did a project on finding the speed of his nerf dart gun.  He used a high speed camera shooting at 240 fps and Logger Pro to determine the firing velocity.  He got some good data:

dart1 redo

 

graphs of nerf dart speed

 

If you are interested, here is his video:   Derek’s nerf dart video

I was shocked when I saw that his results were practically identical to mine!…and Derek wasn’t using the same type of nerf gun.  That has to be partly coincidental.  Physics Professor/Blogger Rhett Allain did an analysis and got a much slower velocity from his nerf gun.  Here is his post about it.

I was also surprised that Derek’s results were a lot more consistent too; his uncertainty was more like +0.5 m/s.  So, nice job Derek and thanks for letting me use your video and results in this episode:)

If you want to do your own video analysis you can use a free program called Tracker.   It does the same sort of thing that the Logger Pro software does.  If you have Logger Pro already (mainly educators I would imagine) but don’t know how to do video analysis yet, you should learn–it is an awesome tool for analyzing motion.  I made this screencast to help my students.  The audio is a little too low, but maybe it can help you get started too.

Summary

I have no idea what happened in my last attempt at finding the dart speed.    Those results now seems way too high.  I must have messed up the analysis.  However, I now feel confident in saying that my nerf gun fires at a speed just under 14 m/s which is about 30 mph.

Now go figure out how fast your nerf gun fires!  Email me your results.