Science Buddies: "Ask an Expert"

I am doing a science project in physics. I am looking at how different angles, 'squats' and 'wall slides' (wall sit) put tension on the knee and the quadriceps tendon. My experiment is slightly different from the normal hooke's law experiment as i am not hanging weights of the spring to extend it. I have developed my model from a science buddies model; i have attached a spring to nylon string which is attached to a guitar tuning mechanism. I have then changed the angle of the knee joint from 180,165,150,135,120,105,90,75,60,45 and each time measured the extension of the spring in order for the leg to remain in position.

Ive looked at tutorials of hooke's law and i think i will need create a graph once i have found F and then the gradient should be 'K'. In order for me to find 'K' i need to first calculate the force 'F' but that is where im confused. I think the force would be mg but as i am changing the angle im not sure if i have to use mg cos (theta) and change theta for each angle to get the different forces, or if i should be calculating the force another way. Please help!! ive spent alot of time building the model and getting to this stage but im now stuck and need help! Thanks for any help you can give.

Hi,

Without seeing your exact experimental setup I cannot give you a specific answer. In general terms, in order to apply Hookes Law you will have to resolve the force vector you are measuring into two components, the forces parallel and perpendicular to the spring you are extending. Hookes Law only applies to the force component in the same direction as the spring extension.

Here is a Science Buddies experimental writeup of a knee extension experiment:

https://www.sciencebuddies.org/science- ... p006.shtml

And a simple Hookes law experiment:

https://www.sciencebuddies.org/mentorin ... p027.shtml

http://www.batesville.k12.in.us/Physics ... Scale.html

Here are tutorial pages referenced in the writeup on force vectors and how to resolve them:

http://www.physicsclassroom.com/Class/vectors/u3l1d.cfm

http://www.physicsclassroom.com/Class/vectors/u3l3b.cfm

http://www.physicsclassroom.com/Class/vectors/U3L3c.cfm

I hope this gives you enough to figure out how to apply it to your particular setup.

Best of luck on the project!

Barrett L Tomlinson