Author: Jonathan Cousins, SEAS’14
As I looked down three stories to a staircase in the Skirkanich building, I was worried.We were about to drop a GI Joe down attached to nothing except a bungee cord made of rubber bands, and only the prediction of our computer program telling us that he would not hit the ground.Our professor turned on the accelerometer, counted down – 3, 2, 1, go! I let go of the GI Joe, and…
Let me back up for a minute. This is a class project for MEAM 348, the second semester junior year lab for Mechanical Engineers.The goal was, given a height and mass of a GI Joe, design a bungee cord that would give him the most free fall time while not hitting the ground and not exceeding 5 Gs in acceleration.My team started out testing individual rubber bands with different masses to see how they stretched.The difficulty with using rubber is that it does not stretch linearly.A spring will deform an amount that is proportional to the force put on it, but rubber bands do not do this.We created a graph of how they changed from our experiments, and used this to try to predict how the rubber bands would behave when grouped together.
After many hours of testing, thinking, and coming up with a model, we finally had something that could predict how far our GI Joe would fall, and how many G forces he would endure.We then started doing drops where we made bungee cords and checked to see that they performed as our model said that they would.There were a few mishaps – strings coming untied, masses flying – but the tests we ran suggested that our model was pretty good.
Then came the demo day.We were given a height much larger than anything we had tested, and we did not know how well our model would do.We had an hour or so to figure out a good configuration and put the rubber bands together – we ended up using 7 strands of 46 rubber bands each. During this process other teams that tested before us kept coming back into the lab, and many of them had stories of their GI Joes hitting the ground, sometimes with a lot of force. We watched a drop right before ours where the GI Joe hit the ground so hard, the accelerometer came off!
Finally, it was our turn.We set up our cord, and attached the GI Joe.The count went, we dropped the doll, and it didn’t hit the ground!We were excited, but we would not know how well it had gone until we looked at the accelerometer data.We checked it, and the G forces were close, but just under 5 Gs.We had done it! But almost immediately our attention turned to the next lab: trying to build and model a cooler that could cool drinks to a specified temperature after a specified amount of time – another daunting task. But for that short time, we knew we had truly accomplished what we set out to do.