In this time of quarantine and isolation, it has been hard
to keep the kids entertained, busy and learning. I have been trying to teach the scientific
process and working though problems to a solution. I am an engineer by career and degree, so
these attributes are important to me. I
saw a plan to make a rubber band gun in a magazine. I didn’t think it would be too hard to create
experiments that would entertain the kids and allow them to look at data to
come up with a solution. This project
was not as easy as I thought it would be.
Here are some of the highlights that I learned.
Setup: I created a
board with 11/32” holes spaced at ¾” from the centerline. I was able to get 10 sets of holes. I drilled 7/36” holes on the centerline and
spaced them 1” apart. I drilled 3/16”
holes in a clothes pin 1” apart and put in 3/16” dowels. Created spacers in 3/16” thick material. Sorry, I did not take pictures of the test
board.
Experiment 1
What height should the rubber band be placed to have optimal
distance? We set the rubber band holder
and the clothes pin distances as constant and raised the clothes pin by 3/16”
spacers. (This was the thickness of scrap I had. 1/8” would also work.) My youngest helps with this part. We had a sample size of 6 at each height. The neat thing about this experiment was
there was definite height that was the optimal height.
Experiment 2
What should the space between the pins that hold the rubber
band be? We set the height at the
optimal height from the last experiment.
Unfortunately, we could not set the clothes pin distance constant. As the pins get closer together the potential
energy of the stretched rubber band is decreased. Yes, I did consider this and had a solution,
but I did not do it. To compensate we
moved the clothes pin back after we moved the rubber band pins in three
times. This did muddy the results. There was a couple of spaces that did create
very consistent results. I did a sample
set of 10 on this experiment because the middle child, who likes science, was
helping me. That was not a good idea
because he had about the same attention span of the younger child. Should have kept at 6 samples.
Experiment 3
How far back should the clothes pin be? The rubber band pins and clothes pin height
remained constant. We were not too
concerned with distance and consistency on this experiment. We just wanted to see what distance the
clothes pin could hold the rubber band without misfiring.
Results
So, with all the data and observations, we determined the
best dimensions for a rubber band gun. I
took some clip art and blew up the back section to trace patterns. The kids and I traced and cut out the guns. Attached the cross piece to hold the rubber
band and the clothes pin. The kids were
excited and the wife not so much. The
oldest decided the ping pong balls were not that impressive so he has tried
shooting other things.
Here are some of my observations on this process. Some are design related and others
scientific.
- Bernoulli's principle – It was actually pretty cool to see this in action on the height of the clothes pin experiment. Depending on the height of the rubber band it would put a top or bottom spin on the ball. Top spin would make the ball just drop. The bottom spin would cause the ball to float. It made sense the optimal height was just below half the diameter the ball.
- Eight-inch rubber bands degrade quickly. It only takes 5-10 stretches of a rubber band before it stretches over 8”. We did account for this and change the rubber band for each experiment. Where this proved to be difficult was experiment 2. The distance that gave the most reliable results was when the rubber band stayed in place on the pins. When the rubber band did not have the tension to keep it in place the results were not as consistent. This was because we would have to put the rubber band after each shot. A little high gave the ball more top spin and a little low and we would have more bottom spin. On the guns we ended up wrapping the rubber band around each pin once to hold it in place
- Ergonomics and gun holding is a pain to figure out. The trigger finger is not the index finger but the thumb. That makes it difficult to hold. It was a lot of cutting. Cardboard only helps so much because it is only 1/8” thick.
- Angling the clothes pin does not help a lot. It actually could create a problem if angled too much; the rubber band might not fire.
- Clothes pins are cheap and break easily. This could be because I made them not well supported and replaceable.
- These guns are super inaccurate and unreliable, but fun and sort of safe to shoot. (doesn’t leave as big of a mark as the rubber band gatling gun; not speaking from experience)
No comments:
Post a Comment