Fred's World of Science: Fun at the U of C

Each year we have a little intracollegiate competition we call the scavenger hunt. My first year here, I helped out a lot. One of the items on the 300+ list was a rope bridge that spans the midway. The rope bridge had to support the weight of at least one member of the team. Our team, the Mathews house team, was the only team to successfully complete this item.
To give those of you who have never been to the U of C an idea of the midway, there is a valley that is 8 blocks long, and one block wide dividing a few buildings (including my dorm) from the rest of campus. It was the place of the 1897(?) World's Fair, held in dear old Hyde Park in Chicago. It has remained a landmark of Chicago; the home to many statues, city park fairs, city skating rinks, etc. It is 208 feet wide, tree to tree, and at max 15 feet below the steet level. There's also little trees that line the edge of the midway. The trees are just big enough to support rope, and you guessed it - me.

I took three pitches of 11mm static climbing rope and joined them end to end with double fisherman's knots. Then, I tied one end to a tree with a figure eight followthrough around the trunk and over a branch to prevent slipping. I stretched the rope to the "pulley" point, and got about eight people to pull on the slack end in order to pull the rope tight enough to hang from it. We couldn't do it.

So, I got my truck, an 89 Bronco ][, and attached the rope to the truck hitch with some shackles, webbing and proof chain. I slowly pulled away, and the rope got tighter and tighter. I pulled until a friend was suspended in mid-air in the middle of the rope. Then, I simply put the car in park, and walked away. I tested the bridge in my harness while my buddy in crime Ian watched carefully. The rope was extremely tight when one of us was on it, and I later calculated the forces (I am a physics major, afterall). Needless to say, the bridge worked perfectly, and Ian went across for the competition. Everybody thought it was pretty cool.

On the left is a picture of me testing out the bridge. On the right is a picture of Ian crossing the bridge in the competition. Isn't physics fun!?

I later calculated that when one of us was in the center of the rope, we were exerting about 3200 N in tension on the rope. This is equivalent to lifting 734 lbs with the ropes. The rope was rated 5000 newtons, so I figure we were pretty safe, except when we bounced a little, or when we were close to either end of the rope. I figure that when we approached the sides, we were exerting close to 5500 newtons, and we were noticing creaking noises from the ropes (eek!).

Who says U of C kids don't know how to have fun???

Copyright 7/29/97 Fred M. Niell, III
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