# Homework Help: Hook's Law

1. Aug 5, 2008

### Gear2d

1. The problem statement, all variables and given/known data

If a person holds a 30cm spring and compress on it with a force of 100 N (where k = 1000 N/m), by how much is the spring shortened.

2. Relevant equations

Hooks ' Law: F = -k* deltaX

3. The attempt at a solution

The answer for this is 10 cm: 10 = -1000 * deltaX

Why doesn't the length of the original spring matter here? Also, how does the "-" sign affect Hook's Law here in this problem.

2. Aug 5, 2008

### Hootenanny

Staff Emeritus
Let's turn it around, why do you think that the unstretched spring should matter? Does an unstretched spring exert a force?

The negative sign simply indicates that the force exerted by the spring is in the opposite direction to the compression/extension.

3. Aug 5, 2008

### Gear2d

What you are saying is that here they are looking for how much the spring was compressed by, not the final length of the spring after compression (which is 20cm?).

4. Aug 5, 2008

### Hootenanny

Staff Emeritus
Yes, the force exerted by a [hookean] spring depends only on the displacement from it's natural length. The 'delta x' in Hooke's law represents the change in length of the spring, the initial and final lengths of the string are irrelevant.

5. Aug 5, 2008

### Gear2d

Thanks Hootenanny