# Homework Help: Hooke's Law Problem

1. Dec 7, 2007

### astro_kat

1. The problem statement, all variables and given/known data
All right I have a graph of a spring being pushed by a person's hand. the hand is exerting force upon a force sensor attached to the spring. (So the force sensor can be measuring the force of the person's hand or the force of the spring). The questions says that the graph should follow Hooke's law, however the graph is linear increasing. How than does it follow Hooke's Law if:
every point on the line is an (x,y) coordinate: (x;-k*x) (where x is position and k is the spring constant) shouldn't the graph be linear decreasing?
2. Relevant equations
F=-k*x
F=k*x
(where -k*x is force of spring aka Hooke's law... and k*x is the force applied by the person)
3. The attempt at a solution
Well i considered two things A) if it was increasing due to the graph's relative position being zeroed at the spring's equilibrium position. And B) that the expirement takes place on a horizontal plane, so without gravity that could be what's throwing me off. :yuck:

2. Dec 7, 2007

### pooface

If 'x, displacement' is represented by x on the graph and y = -k*x

The more force exerted the higher the X value and if X increases then -k*x should increase as well.

I am not sure what you are asking exactly.

3. Dec 7, 2007

### Shooting Star

Hi astro_kat,

You are just plotting the magnitudes in the graph. That's why it's linear increasing.

You may also want to ascribe proper directions to the force and x. If you say that the force is positive when the hand is pulling, and x is positive when it's stretching, then also the graph is linear increasing.

4. Dec 7, 2007

### astro_kat

That's not exactly what i meant. why isn't it decreasing? Let's say x=.5 meters. than the coordinate pair would be:
(.5, -k[.5]) which means that the y coordinates should be negative when x is positive and vise versa, therefore a negative slope should result, right?

5. Dec 7, 2007

### Shooting Star

You are correct. Well, as I said before, often people plot magnitudes in graphs. It may be common practice.

6. Dec 7, 2007

### rl.bhat

That's not exactly what i meant. why isn't it decreasing? During pushing or pulling the force sensor should show increasing reading. In the case of pushing the x is negative and graph is in the second quadrant. While pulling x is positive and the graph is in the first quadrant. In both the case it is increasing.

7. Dec 7, 2007

### astro_kat

Magnitudes=absolute value of Force.... why didn't you say that. way to be ambiguous!:rofl:

8. Dec 7, 2007

### Shooting Star

Think of the magnitude of my absolute valuelessness...