Science Buddies: "Ask an Expert"

I am doing my science fair experiment, looking at a model of the patellar tendon. I am using a science buddies model to begin my model. I want to actually test the length of the spring (representing the quadricep) versus the amount of force put on the patellar tendon. I am using F = -kx. I understand how to find x, but I do not understand how to find the constant, k. I know F = ma, but I don't think I can use gravity for a with the model I am using from science buddies. I have already made the model, but I am unsure how to find F if I don't know how to find k. Please help! I have put alot of time in researching the anatomy of the knee and Hooke's Law, but now I am stuck!! Thank you for any help you could provide.

Hi dhart.

You will have to run an experiment and graph the results to find k. If you do a search on google for "Hooke's law calculate k tutorial," you will find several sites and documents that explain how to run this procedure. Good luck and please write back if you have any more questions. Good luck!

Hi!
Well if you are hanging the spring straight down, and there is nothing else forcing the spring down but gravity, you should be able to set F = weight = mg, where m is the mass of the object that is on the spring and g is the gravitational constant 9.8 m/(s^2). you can then set F = -kx = mg which should help you out. Hope this helps!

dcnick96 wrote:Hi dhart.

You will have to run an experiment and graph the results to find k. If you do a search on google for "Hooke's law calculate k tutorial," you will find several sites and documents that explain how to run this procedure. Good luck and please write back if you have any more questions. Good luck!

Thank you for your reply! I have looked at tutorials online, but I am actually attaching the spring to a guitar tuning mechanism (not hanging wights on it) and then looking at the different forces when the spring is stretched. I stetch the spring by putting my mechanical knee at different angles. The tuning mechanism is then used to keep the knee at that angle. In order to calculate k, I need to know the force exerted, but that is what I am actually trying to find in this case.

Thank you for your reply! I have looked at tutorials online, but I am actually attaching the spring to a guitar tuning mechanism (not hanging weights on it) and then looking at the different forces when the spring is stretched. I stretch the spring by putting my mechanical knee at different angles. The tuning mechanism is then used to keep the knee at that angle. In order to calculate k, I need to know the force exerted, but that is what I am actually trying to find in this case. Should I do a simpler experiment to find k first? After finding k for the spring, then I could reattach the spring to my mechanical knee and use the k I found earlier to find the different forces (F) at different angles. I am not sure if this is acceptable - would the k remain constant for the spring even if it is put in the mechanical knee?

k is a constant for "spring stiffness," which should remain constant, regardless of the type of test you are running, for a spring. Therefore, your idea to do an easier experiment to calculate k is a plausible one. What you can't do, however, is use one value of k for different springs, since the "stiffness" of each spring may be different. Hope this helps and good luck!