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 Formula

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.

dart with dots

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.

Results

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).

*UPDATE*

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.

*Newest Update*

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.