Science Buddies: "Ask an Expert"

I'm doing the "Deep Knee Bends: Measuring Knee Stress with a Mechanical Model" project, and I built the entire knee except I can't attach the guitar string. Whenever I attach it, I can bend the knee at any angle and the hip will stay in place even if there isn't any tension on the string. Then when I push the hip back up and everything is straight, the string is loose. How do I attach the string? Nothing I try is working, and the procedure from Science Buddies didn't go into detail about how to attach the string. I'm very confused.

Thank you in advance!

I'm begging anyone out there that can help me to please do! I am lost and have nowhere else to turn. I really need to do well on this project and so far this is the only thing that has stumped me in constructing the model.

Any help would be greatly, GREATLY appreciated!

Guys I'm honestly begging you. There has got to be someone out there that can help me with this. I see that 14 people viewed this! I see you! This may be humorous to you people out there but I honestly need help! Trust me, this is my last resort. If you ask me, this message board is useless. I wish I could "Ask an Expert" directly! NO ONE IS GOING TO ANSWER MY POST! THIS IS NOT HELPFUL TO ME! And if a moderator deletes this, so be it. I'm just stating the truth.

Anyway, please help me if you can. This is agonizing.

Hello,
I am sorry no one has responded yet. It may be that no one who has come across it has been able to answer your question. Sadly, I believe I'm in the same predicament. I will, though, post a link to this in our separate forums to try and reach other experts. Hopefully someone with more expertise than I've will assist you.

Please try to be patient, as we are all volunteers, and we all lead some busy lives and right schedules, but I'll try increasing the traffic to this post and see if someone who knows what they're taking about can help you.

I hope this helps a little,
Meg

Hi:

I see two questions you're asking:

I can bend the knee at any angle and the hip will stay in place even if there isn't any tension on the string.

.
It sounds like there is too much friction in your setup. Before even thinking about the guitar string, be sure that your apparatus can easily travel up and down in the tracks. If not, find where it binds and fix it. From reading the experiment, it looks like the "hip" piece should slide all the way to the bottom (and bend the knee completely) when the string and spring are not connected (due to gravity).

I can't attach the guitar string.

According to the experiment plan, the guitar string is attached at one end to the spring and at the other end to a guitar tuner. Do you have all of these pieces attached to the apparatus? The way I understand the project, you put the "hip" at different heights from the ground and adjust the guitar tuner to stretch the string to provide exactly the right amount of force to hold the hip in place. Is this how you understand it? let us know.

Keith

First of all, THANK YOU BOTH SO MUCH!!!

Keith:

Yes, that's how I understand the experiment. I knew that the hip should have only stayed at a certain angle if the guitar tuner was turned to cause tension on the string, but, as you suggested, the hip was staying in place at different angles on its own, possibly because of too much friction. My model is not with me now, but the leg was able to stand upright on its own, as I recall, because it was resting on the piece of wood representing the foot. Now that I think about it, I suppose that the hip should have been falling due to gravity anyway. I'm not sure how I can reduce friction in the metal track where the casters roll. Also, the experiment called for casters, but I'm using casters that swivel/rotate. If anything, I thought this would decrease the friction in the track and allow the hip piece to move more freely. Do you think this is negatively affecting the model?

*Tumbleweed blows by* Hey guys. I'm still here. And I still don't know what to do. The stress levels are increasing by the minute. If I have to restart this project, I will fail this semester. Which means I'm done for. Say goodbye to an Ivy League. This project is eating away at me from the inside out. Please. I'm begging anyone out there to find a place in their heart to help me. I'm just a hardworking student that is in quite a predicament right now.

Thank you.

Hi, I am almost complete constructing the model. There are 2 springs given in the materials. 1 spring 2-1/2 in long, 5/8 in diameter, 0.072 in gauge and
1 spring 2-1/2 in long, 1/2 in diameter, 0.047 in gauge
I have used only the 1/2 in diameter spring to attach the guitar string from quadricep spring to the tuning mechanism. What is the other one used for and where?