Science Buddies: "Ask an Expert"

My project is based off the science buddies project "Deep Knee Bends: Measuring Knee Stress with a Mechanical Model". The issue at hand is that the experiment is flawed. In the project's materials, a nylon guitar string is suggested, yet (after consultation with the local high school physics teacher) these strings can become deformed and permanently stretched. Second of all, the hip piece of the mechanical knee model is causing friction when sliding up and down, providing me with a range of answers for every angle, resulting in differing measurements in every trial, skewing my data. Now, the aforementioned physics teacher helped me create a formula to calculate the force, but this was quite different from the original readings, and science fair is only two weeks away. Should I present my flawed data as real? Do I show them the flaws and illustrate my failed attempts at fixing it? Is there a way I can get rid of the friction and take more accurate measurements? Any help would be appreciated. Thank you.

I'm sorry to hear you're experiencing so many difficulties with your experiment. But, actually, this is REAL SCIENCE. We form hypotheses and dream up experiments to prove them, but things don't always work out the first time. When this happens, we learn from our mistakes, correct them, and learn even more on our next try.

It's unfortunate you have only 2 weeks until your science fair. This gives you little time to correct your problems, but all is not lost.

First, the excess friction you are getting most likely has something to do with the way you constructed your model, or, perhaps, the materials you selected to build it. I've just looked over the description of this experiment, and see it is a fairly complicated model of the knee. Congratulations for taking on a challenging project like this!

But problems like this often show up when we design and run experiments. You didn't fail, you have just been given an opportunity to learn about new things you didn't plan on learning.

Regarding the stretching of the string... I'm sure that when a nylon string is held in tension for a long time, like weeks or months, when strung on a guitar it does stretch - and maybe a lot. Nylon is a stretch material. That's why mountain climbers don't use it for their ropes. This is a phenomenon known as "creep". But the key here is that is takes quite a bit of time for it to stretch. Over the course of time it should take to acquire the data for your knee experiment, this should not be so much a problem. If it is a problem, you could use a thin braided cotton shoe lace, or even a length of braided steel picture hanging wire.

The friction problem you noted is not uncommon in a lot of mechanical devices. It's important that you realize this is a problem, and I realize you probably don't have enough time to rebuild it, but you might try some sort of lubricant to minimize the amount of variability in your data you are experiencing.

If this doesn't work, then you'll have to do the best you can with the data you have. If you run many trials, and then apply statistics to the data, such as averaging it, you can smooth out a lot of that variation in your analysis.

Presenting your flawed data at your science fair is completely acceptable, as long as you indicate it is flawed and that you can explain why it is flawed. Any good science fair judge will then ask you what it has taught you, and if you are going to do your experiment again, what would you do differently. So, you might want to think about that.

I hope this helps, and that this experience has not discouraged you from becoming a scientist or engineer. I am an engineer, and have been for more than 50 years. I love it. Although I don't like making mistakes in my engineering work, I have made plenty over the years, and have learned from every one of those mistakes. I'm a MUCH better engineer now than I ever would have been if I didn't make those mistakes and learn from them.

Good luck!