# Block hung from vertical spring

1. Mar 30, 2017

1. The problem statement, all variables and given/known data
A block is hung from a vertical spring. The spring stretches (h = .0650 m ) for a particular instant in time. Consider the earth, spring, and block to be in the system. If m = .865 kg and k = 125 N/m, find the change in the systems potential energy between the two times depicted.

2. Relevant equations
(1/2)ky^2(final) - (1/2)ky^2(initial)

3. The attempt at a solution
My book actually doesn't have an answer for this question. I assume the equation I wrote above is the one I use.

Plugging in I got:

(1/2)(125)(-.0650)^2 - (1/2)(125)(0) = .2640625

does the answer I got seem correct to everybody?

2. Mar 30, 2017

### haruspex

The question statement is unclear. Is the block attached to the spring with the spring relaxed, and then released? The text implies a diagram, but you did not post one.
It asks for the change in "potential energy", but you only calculated a change in elastic potential energy.

3. Mar 30, 2017

### Comeback City

Is there any gravitational potential energy involved? The question seems to imply that there should be.

4. Mar 30, 2017

Sorry. Yes there is a diagram. It starts with a block at (0,0) and then the block goes to (0, -.0650 m)

5. Mar 30, 2017

### haruspex

So what is the total change in PE?

6. Mar 31, 2017

Is it .2640625 Joules?

7. Mar 31, 2017

### haruspex

No, that's just the change in elastic potential energy. Reread posts #2 and #3.

8. Mar 31, 2017

http://imgur.com/7OmNvFY

Here is the full question. It is question number 26. The section its under is called "Elastic Potential Energy."

Sorry I didn't make these clear in the OP

9. Mar 31, 2017

### haruspex

No, you had it right in the OP. It says to treat the Earth, spring and block as The System, and asks for the change in potential energy of The System. It does not restrict it to elastic potential energy. If it did, there would be no need to consider the Earth as part of The System.

10. Mar 31, 2017

I see. So what I do now is add my answer in the op, with (Gravitational potential energy final - Gravitational potential energy final) to get total PE for the system?

11. Mar 31, 2017

### haruspex

Yes (except you didn't mean "final" twice).

12. Mar 31, 2017

Wow I was sure I wrote (Gravitational potential energy final - Gravitational potential energy initial,) lol.

So the key with this problem is understanding there is 2 sources of energy and to add these up to get the net potential energy, right?

13. Mar 31, 2017

### haruspex

It's that there are two forms of potential energy involved.