# Homework Help: Hooke's Law Lab and theoretical y-value! Really need some help!

1. Nov 12, 2009

### exparrot

I did a Hooke's Law lab in class last week and one of the post-experiment questions asks to explain why the graph made should be a straight line and what should the slope and y-intercept be in terms of quantities in this lab? I know that Hooke's Law demonstrates that the amount a spring is stretched (displaced) is directly proportional to the restoring force, thus the line of the graph would be linear. For my graph, I have the length of the spring in meters (x-axis) vs. the F (m*g) in newtons. The slope would be the k constant and the y-intercept, I assume, would be 0 as if you have 0 displacement, the resultant restoring force would be 0. Okay so I answer that just fine and move on to question 2. Question 2 asks me find the percent difference between the experimentally obtained y-intercept and the theoretical value obtained in question 1. My experimentally obtained y-intercept is 0.0014, and if I assumed correctly in the previous question, my theoretical y-intercept is 0. How would I go about finding the percent difference? I would have 0 in the denominator and I wouldn't get a percent difference, although there is clearly a small difference. Is my theoretical value for the y-intercept correct? I would really appreciate the help! Thanks!

2. Nov 13, 2009

### Delphi51

This suggests that you used not the spring stretch x, but its original length L plus the stretch x. If so, your y-intercept will not be zero.

But most likely, you used the extension x on the horizontal axis and everything you wrote makes sense. If you predict 0 and get 0.0014 the % difference between them does not make much sense, but I would use 100% if I had to have such an answer.