Finding x for a spring using energy equations

  • Thread starter cbasst
  • Start date
  • #1
33
0

Homework Statement



This isn't a homework question, just something I've been thinking about. If given a situation in which mass m is hung on a spring with constant k, I know you can set the spring force equal to the weight of the mass to find how far it stretches. Is there a way to find how far it stretches using energy equations? It seems like it should be doable, but I can't figure out how.

Homework Equations



w = mg
F = -kx
ΔUg = mgh
Us = kx2/2

The Attempt at a Solution



As I said, setting mg = kx can solve for x. How do I do it with energy though?
Ug = Us
mgx = kx2/2

But now I get a different answer for x. What did I do wrong with the energy equations?
 

Answers and Replies

  • #2
Infrared
Science Advisor
Gold Member
929
518
The energy approach will give you the farthest the mass on the spring will fall. This is not the equilibrium position. Think about it, if I let the mass drop, then when it passes the equilibrium position, it will not have zero velocity.
 
  • #3
33
0
Ah. This makes sense now! Thanks!
 

Related Threads on Finding x for a spring using energy equations

  • Last Post
Replies
4
Views
2K
  • Last Post
2
Replies
30
Views
6K
  • Last Post
Replies
4
Views
1K
Replies
5
Views
4K
  • Last Post
Replies
5
Views
4K
  • Last Post
Replies
1
Views
2K
  • Last Post
Replies
1
Views
3K
Replies
2
Views
471
Replies
0
Views
3K
Top