Posted by **Janice** on Thursday, June 23, 2011 at 6:28pm.

So I did in an experiment and the results I got were the potential energy when the spring compresses is 0 and when it stretches is 1.42. This spring is vertical and has a 250 gram weight attached to it. I was very confused when i got these results because I thought the potential energy is higher when a spring compresses and when it stretches the potential energy turns into Kinetic energy.

- Physics -
**Damon**, Thursday, June 23, 2011 at 6:32pm
When there is no compression OR tension on the spring, usually we call the potential energy zero.

If you either compress it or stretch it, the potential energy increases.

U = (1/2)k x^2

that is + for -x and for +x

The kinetic energy is only when the mass is moving

Ke = (1/2)m v^2

- Physics -
**Janice**, Thursday, June 23, 2011 at 6:37pm
So my results are incorrect right? The PE should not be 0 when compressed.

- Physics -
**bobpursley**, Thursday, June 23, 2011 at 6:44pm
http://www.youtube.com/watch?v=sTsUx-6CflIhttp://www.youtube.com/watch?v=sTsUx-6CflIhttp://www.youtube.com/watch?v=sTsUx-6CflI

- Physics -
**Damon**, Thursday, June 23, 2011 at 6:48pm
250 gram = .25 Kg

weight = m g = .25*9.8 = 2.45 Newtons

That stretched the spring 1.42 cm?

1.42 cm = .0142 meters

so k of spring = force/elongation =2.45/.0142 = 172.5 Newtons/meter

Potential energy stored in spring = (1/2)k x^2

=(1/2)(172.5)(.0142)^2 = .0174 Joules

Potential energy lost by mass going down in gravity field = m g h

= .25*9.8*.0142 = .0348 Joules

The mass lost more potential energy than the spring gained (twice as much)

So we have .0174 Joules left over when the mass stretches the spring. That energy goes into kinetic energy. The mass falls through that .0142 down point and keeps going for another .0142 meters where it comes to a stop, then bounces back up again to the start, again and again :)

- Physics -
**Damon**, Thursday, June 23, 2011 at 6:52pm
The nasty thing about potential energy is that you can define zero anywhere at all. What matters are differences.

- Physics -
**Janice**, Thursday, June 23, 2011 at 6:54pm
Thanks guys info really helped!

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