# Homework Help: What is the spring constant?

1. Feb 27, 2008

### mortho

1. The problem statement, all variables and given/known data
An elastic cord is 67 cm long when a weight of 57 N hangs from it and is 80 cm long when a weight of 79 N hangs from it. What is the "spring" constant of this elastic cord?

2. Relevant equations

F=-Kx

3. The attempt at a solution

I substituted the numbers into the formula for each set, like for the 0.67 m, i got 85.1 N/m and for the other i got, 98.8 N/m and then i averaged the two because i didn't know what else to do, so i got 92.0 N/m for the average but it's wrong. Please help me!
Thanks

2. Feb 27, 2008

### blochwave

Remember in that equation that x is the distance from equilibrium

You don't know how long the cord was when there was nothing on it, so you're not gonna have it but you're given two cases, so eventually it'll cancel out

3. Feb 27, 2008

### uber_dzl

i have a similar problem so i didnt want to start a new thread, i hope there arent any feelings of thread jacking, but my problem is that i have the force and the k constant and an original length but i need to find the final length. would it be the force added to the k * x or multiplied like this: (mg)*k(x_2-x_1)

4. Feb 28, 2008

### uber_dzl

i sense that i am completely off....i will rethink this and come back

5. Jun 7, 2010

### FiveAlive

Yo, to calculate the spring constant you just have to use the change in mass.

76N-57N=19N

the change in distance (delta x) is 80cm-67cm=13cm

change cm into meters dividing by 100

13cm/100=0.13m

now plug n chug

19N=-K*0.13m