Block hung from vertical spring

[Rijad Hadzic]

Homework Statement

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.

Homework Equations

(1/2)ky^2(final) - (1/2)ky^2(initial)

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?

 

[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.

 

[Comeback City]

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

 

[Rijad Hadzic]

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.

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

 

[haruspex]

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

So what is the total change in PE?

 

[Rijad Hadzic]

So what is the total change in PE?

Is it .2640625 Joules?

 

[haruspex]

Is it .2640625 Joules?

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

 

[Rijad Hadzic]

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

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

 

[haruspex]

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

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.

 

[Rijad Hadzic]

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.

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?

 

[haruspex]

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?

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

 

[Rijad Hadzic]

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

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?

 

[haruspex]

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?

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