# Spring Problem (Hookes law)

1. Jul 24, 2017

### I love physics

1. The problem statement, all variables and given/known data
A block of mass M is hung on a vertical spring.
We have to find spring constant k, x (displacement from mean position) given.

2. Relevant equations
1/2kx^2 = mgx
mg=kx

3. The attempt at a solution
When i conserve energy,
1/2kx^2=mgx
⇒ k= 2mg/x

But when i use mg=kx,
I get k=mg/x

But mg/x≠2mg/x,

2. Jul 24, 2017

### .Scott

You don't need to compute energy here.
Just use Hooke Law directly.

What force is M applying to the spring?
How far is this causing the spring to stretch?

The assumption here is that the system is stable at x.

Last edited: Jul 24, 2017
3. Jul 24, 2017

### I love physics

after that it is given some velocity v from down, i need to calculate how high will it go.
i used 1/2kx2=1/2mv2 is it correct?

4. Jul 24, 2017

### haruspex

Is that based on some reasoning or just a wild guess?

If it rises above the equilibrium point by a maximum of h, what are the changes in
1. KE
2. GPE
3. Elastic PE
?

5. Jul 25, 2017

### J Hann

Consider the situation of the mass moving horizontally along a frictionless surface.
What is the difference in the two situations?