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:
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:
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.
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.
March 4, 2014
Who can resist a Nerf gun? You just can’t help but pull the trigger if one is nearby. But have you every wondered how fast the darts are actually moving? Well, in this episode I answer that.
Episode 7: How fast is a Nerf Dart?
Speed = distance/time
I could have just measured the total sideways distance the dart traveled and divided it by the time it took to hit the ground. But that would have given me the average speed of the dart while in the air. Since our Nerf darts are made of foam, the air slows them down while they are flying. That means that they are moving faster in the beginning and slower at the end. That makes the averages speed somewhere in the middle. But I want to figure out the speed (or velocity) that the Nerf gun fires them. So, that’s why I need to use the slow motion video. That way I could see how far the dart was traveling in the first 30 cm or so before the air had a chance to slow it down much. I used Logger Pro to analyze the slow motion video. Here are a couple of screen shots from the video analysis.
In the Logger Pro software, I clicked on the tip of the Nerf dart for each frame of video. So the blue dots represent where the dart was for every video frame. It does look like those dots are slightly closer toward the end. That would mean that the dart isn’t travelling quite as far between each frame of video. Or in other words, air resistance slowed the dart down a bit.
So I found that my nerf gun shoots the darts at around 16 m/s or 35 mph. (I did find 19m/s when I analyzed the video one way–but I think there must have been an error with that).
I revisited this. You can find the most recent experiment here.
The short version of my second attempt is that I measured the speed with photogates and got just under 14 m/s or 30-31 mph. I then tripled check it with a radar gun and also got 30-31 mph. In addition, my student measured his own nerf gun’s speed and also came up with just under 14 m/s.
So, I’m feeling a lot more confident about 14 m/s or 31 mph.
The newer episode also talks more about uncertainty in my measurements, so if you’re interested check that out with the link above.
I tried a different gun, the Nerf Sharpfire. It was a little slower at around 11 m/s. I also investigated how air resistance slows the dart down and what percentage of its energy is “lost”. It was pretty interesting. You can think about air resistance being like riding in a car and sticking your head out the window–the faster you go the more the air pushes back on you. I shot a whole video lesson around the concept of nerf guns and air resistance for a learning website called Curious.com. Here is a short clip from that lesson.
It was a lot fun making the lesson and I was surprised at the results. You can sign up for a free trial subscription at Curious.com and watch the lesson. If you like it then watch all 12 of the video lessons I made about Energy. They are a great mix of concepts, demos, experiments to try and some cool DIY projects to build as well (like racquetball poppers and rubberband cars). Here is the link to the lesson: How to Calculate Air Resistance. Check it out:)
Next, I want to test this little speed demon (the Elite Firestrike)…my son got it recently and it seems super fast.